Sometimes the challenge isn't finding fun so much as pleasing everyone who comes. If your guest list includes kids with diverse interests, consider an activity center where everyone can choose their favorite fun pastime. In nearby MD and VA, you'll find venues offering go-kart racing, miniature golf, laser tag, batting cages, and arcade games all in one venue.
If you'd rather handle the activities and decorating yourself, why not reserve the private dining room of a Washington, D.C. restaurant instead? Many allow food from without but also have party packages for special events, like sweet 16 parties. You're then perfectly set up for food, fun, and gift exchanges indoors!