OP here — thank you for these awesome ideas! Way more useful than what I was finding on google

We did cabin john train room and train ride when our son turned 4 and it was really fun and not too expensive. I think $200 or so for the room, and that includes tickets for the kids to ride the train. I think the website says they limit the # of tickets to 20, but the guys who were operating the train the morning were there could not have cared less and everyone rode - including parents - for no extra cost. The nice thing is that the playground is there as well, so if you want the fun to continue after 2 hours, you can.

Fun parties we've been to included local parks, museums, nature centers, and planetaria. We've also been to parties at rec centers, with a variety of activities, usually of the arts-and-crafts type.

Apparently little kids love the trolley museum, and their party room would be big enough, but it may be too far out.

Anonymous wrote:If you live near a spray ground, that could be fun. I'm thinking about doing my daughter's birthday at the Lee District spray ground. You can rent the picnic pavilion if you get on it early (last year, the reservation calendar filled up before I got to it), and the spray area is gated, with an awesome playground right next to it.

+1. Did this for our 4yo DS and it was great.

A local park or public nature center always works. Sometimes you can use pavilions without reserving them and paying the fees.