I've looked at their website, and can't tell if it is only for special events, or if you can just show up and play. Also, do you think at two years DD is too young to enjoy what they have to offer? TIA!

On our way to 3 under 4!
DD1 1/22/09
DD2 7/16/10
Baby Boy Due This Summer!
Re: Has anyone been to the Bethesda Playseum?
I've been and yes, they do open play. However, I would call to double check before heading over that the time you are going is for open play.
We went on a Sunday and it was pretty crowded. They have lots of little rooms and fun activities for the kids but it's a tight space so if you can go on a weekday, that would probably be better than a weekend. Try to limit the amount of stuff you bring because there's not much of a place to put stuff. They just have one small area for coats and it gets crowded. Also, as soon as you enter, you have to go straight down one flight of stairs so if you can leave the stroller in the car, it may make things easier.
It's definitely something that a 2 year old can enjoy. They have a mini grocery store that my daughter really enjoyed. They also have a dress-up room, a room with animals, a kitchen/cooking room, etc. It's definitely worth a visit.
We've been a bunch of times, and both of my kids love it...they have stuff for all ages. As far as I know, it's always open play, though on occasion they have special events (like a speaker or book signing) that are held in the back room. I've always gone during the week and never had a problem with it being so crowded that my kids can't play with what they want.
They have an art section that's great, though if you think your DD would like it, put her in clothes you wouldn't mind getting dirty. They have smocks and covers, but somehow mine always manage to get paint or whatnot on themselves.