Got back from the trip and I'll just recap what we did in case it's helpful for others down the line. Again, this is for a family trip with two kids under 5 (but pretty much the focus for me was getting the near 5yo out of the house). In Stow, MA area with a rental car.

First, Boston Logan Airport

First, Boston Logan aiport was great. When we arrived and took the airport shuttle to the car rental center, we accidentally left the travel crib on the bus. We had reserved the car ahead of time, which made it pretty fast, but we were loading the car up before we realized we were a bag short. I sort of assumed we would just be buying a new crib, and that's pretty much what the car guys were implying, but while looking through the main lobby area I glanced at the shuttle drop off curb and thought "maybe if i just wait long enough the bus will circle around again and it'll be there". So I went out, and popped into the first bus in line just to check, and it was our bus! The driver had actually noticed the bag and sat for more than 20 minutes just waiting on me to come back. So that was awesome, and I thanked her profusely.

Another point for the Boston Logan was on our return trip, there was zero security line wait. On a monday at 9:30am. No line to drop off bags, no line to go through the scanners, and when one of our bags had to be pulled aside due to a forgotten set of cutlery (camping fork and butterknife) they basically checked it right away, stared at the knife for like 15 grueling seconds, and then put it back and let us go. It was the fastest american airport experience I've ever had.

Western Suburbs

Discovery Museum - Acton

Excellent museum. I guess last year or 2024 they won some big museum award and I think they deserve it. Every exhibit was operational, nothing broken, which for kid-oriented museums is a feat, in my experience. It was kept neat, too. Things that needed to be staffed were staffed. I noted in my original post that I didn't like "big empty museums", and this was the opposite: small, but densely packed. It was "information dense", too, with good signage about concepts. It had an excellent infant area, and I'd say the upper-bound on age was probably like...10? I don't know, that's a little out of my wheelhouse, someone with older kids could probably better say. We were there on a cold day but still enjoyed the outdoor treehouse area + the trails in the woods that extend from the museum. It was difficult to get my kid out of the museum after about 5 hours there (we did a lunch at Not Your Average Joes down the road and then came back). One note: it's very possible to get wet in the water area.

Butterfly Place - Westport

A small butterfly "greenhouse" where you enter a half-dome-like atrium filled with free flying butterflies. There isn't a lot to do here, but we found that if you just find a place to sit and maybe where a colorful shirt, you can have up to 4 butterflies land on you at a time. Unfortunately, it was me wearing the colors, which bred some jealously in the kid before I transferred my sweater to her. We spent about 3 hours here, which I think as probably longer than anyone else there. Kid loved it the whole time. She is very social and pretty much became a tour guide. There are some awesome stick bugs there, too. Good place to hand a phone to a kid and let them take pictures for awhile.

Castle-In-The-Trees Playground - Littleton

Spent about 3 more hours playing here after the butterfly place. Based on reviews from locals I talked to earlier in the week, I was expecting a little more, but it was still a good playground. I have a lot of opinions about playgrounds but I don't want to dive into that right now. But there were a lot of kids at this one even on a weekday (it was maybe spring break?). I assume this changes later in the season, but there was no trash can or bathrooms here, which was kind of a bummer. If you are in the area with a kid, this a good free place to check out.

Concord & West Concord Libraries - Concord

Both libraries are gems, as far as I can tell. The main Concord library has a cool historical adult area, a "hip and with-it" teen room, and a new kid wing with backyard play area. The librarians were very nice (they almost always are, so no surprise there) and helpful, and it was poppin' with kids, moms, and even some dads. Lots of toys, quiet areas, a makerspace.

West Concord library is much smaller, but everything above applies here except the makerspace. I heard good things from other patrons about other local libraries like in Lincoln, for instance.

Walden Pond - Concord

I've been to Walden before, and it's one of my favorite parks anywhere, partly because it's really nothing more than just a really nice pond, with a nice beach, surrounded by nice woods. This trip I was with a kid who usually won't walk anywhere unless there is some snack at the other end, and it was fairly cold. But earlier I noticed the book Walden just by chance as we left the library (in their book sale area) and showed it to my kid, which then made the trip to the actual pond much more interesting for her just because it seemed "famous" or something. And she walked around the whole pond. It's just nice woods. Visitor center docent/employee was very engaging and they had some kid stuff there, too. They will have a new beach shelter this summer. One extra tidbit: my kid peed in the woods about 40 feet from site of the original Thoreau cabin (after inquiring about bladder status for 5+ hours at this point and being given firm negatives).

Codman Farm - Lincoln

This is a community-owned farm that the city of Lincoln was given a few decades ago. They have a hired farmer that manages it, and volunteers help with certain things. It has veggies but also raises chickens, cows, pigs, goats for meat which I believe is butchered there on site. They have a market there on site where you can buy the goods they make. It's an event space, too, and I heard they are soon reconfiguring the whole barn for more event space, bigger store, etc. The cool thing about this farm is that you can walk pretty much anywhere outside except the animal pastures. And it's free. If you want a little more of a "refined" farm experience and pay for it, check out Drumlin Farm nearby.

Boston

Childrens Museum

It's better than the Saint Paul Childrens Museum, but Discovery Museum was better, in my opinion. Boston CM does have lots to do, though, and is very lively. The main climbing structure is quite something, but I was very nervous my kid would climb high enough to get scared and require me to go up into it-- and I do not think that even as a slim guy I could manage to do, the gaps are very small, especially with so many kids in it. Some exhibits are a bit worn (there is a "home depot" tools area that is basically garbage at this point), and there is quite a bit of "scenery" aka stuff that looks fun but really has no interactive or educational component to it. And, I think for my kid at least, we would have been just as happy at the excellent and free playground just down the boardwalk from the museum (Martin's Park, I think).

Modern Pastry Shop

Kids like whipped cream filled cannoli with chocolate chips, who knew?

Final notes

The Stow/Acton/Maynard area is really pretty, even in the drab early spring months. For someone from the midwest, it's interesting to see the little family farms mixed in with small towns, big and small houses, woods, creeks, ponds, etc. The driving was easy, too. Looking at maps is a little weird because of the adjustment needed when looking at distances (midwest proportions vs massachusetts proportions).

One downside is that it's expensive out there.

Thanks for the suggestions I got from this thread, but I also got a ton of info from asking the librarians but especially just other parents (and their kids) about places to go. Everyone I met in the western suburbs would inevitably suggest the Discovery Museum first, but then I got a range of responses. I unfortunately didn't write them all down, but I mostly just wanted to share the experiences we actually had, not places we "thought about" going to. Overall, would definitely go again and my kid has added it to the long list of places she wants to live when she grows up.