Video-games: Video games are a great outlet for those in a wheelchair. Especially for those who are unable or unwilling to play sports, video games can give wheelchair users a chance to scratch their competitive itch. Specifically, the Nintendo Wii is excellent!
Camping: Everybody loves the outdoors, and wheelchair users are no exception. Most campsites have wheelchair accessible spots just make sure you call ahead. Possible fun activities include fishing, hiking (if there are accessible trails) and swimming.
Art: Wheelchair users have a unique vantage point of the world, and it can often lead to great photography. Likewise, many people in wheelchairs are forced to adapt their other senses, and many develop unique artistic talents in the areas of painting, drawing and music.
Traveling: If you took a poll of wheelchair users, a majority would likely mention traveling as their favorite activity. Depending on where you are traveling, make sure you plan ahead and do research on possible Travel Wheelchair.