Exploraworld in Columbia is a huge indoor play area that I always think of as being geared towards younger children (preschool, kindergarten) although I've seen older children there having a ton of fun too. The building is warehouse sized, and you can see almost the whole space easily (no blocked vision--great when you are trying to keep track of children). The space is divided into individual play areas, with a different theme in each--there's a play grocery store, dress-up area, sand pit, ball pit, climbing area, arts and crafts, and their piece de resistance: a real fire engine the kids can climb on.
While a lot of this can be duplicated at home for much less than the entry fee to Exploraworld ($8.50 for most children, although you frequently can get discount coupons in Baltimore's Child magazine), there's something very appealing to the large size and scope of Exploraworld. In the dead of winter, it's a fun place to run around. It's also a perfect place for playgroups to get together or to hold birthday parties--kids can have a lot of fun playing together without the cramped space of home or the mess to clean up afterwards!