Annie's playground in Fallston used to be free to reserve the pavilion, but books up at the beginning of the year. So call immediately if that one interests you.
Honeygo Run is a nice park, but gets crazy crowded on weekends and parking is an issue if there are sports games going on.
Double Rock in Parkville is nice and has lots of options to accomodate different size groups. We had a party at Belmont Park, which is a small park in Parkville. I like Belmont because they still have swings, parking is plentiful and close by, and it's rarely crowded. Belmont doesn't allow alcohol, and Double Rock does.