At my last company we did TONS of fun stuff that everyone could choose to partcipate in (their choice). We had themed Potlucks and had all employees vote for the best dish, most creative dish, etc. - the winner(s) would get a gift card to a popular store (Target, Best Buy, etc.) and we even gave away baseball tix in conjunction with our Cubs vs. Sox Potluck! We were right outside of Chicago so 1/2 of our staff were Cubs fans & the other 1/2 were Sox fans - everyone was also allowed to wear their favorite baseball jerseys, jeans & hats! It was great! We had huge Halloween parties too (no alcohol of course) which included the themed Potluck lunch, decorating contests, costume contests, etc. Family members were invited to come for the Potluck lunch & tour of the "haunted office"! Same thing with the prizes - usually a gift card but sometimes tix to sporting events (Cubs/Sox games, Bears games, Bulls games, etc.) or even an extra Vacation Day for the calendar year. Everyone really got into it & had lots of fun - and the playing field was equal as the "contests" were not about work related things (likes sales, collections, etc.). Another fun one was when we had everyone bring in a baby picture of themselves, photocopied them in black & white (to make it trickier), hung them all up on the wall with a number attached. Then everyone had to try & match up the employee with their baby picture! We actually had someone get 100% of them correct & she (obviously) was the winner! We also had Company BBQ's - one time we even rented a dunk tank & each member of management had to sit in it for 10 minutes while various employees tried to dunk them. Another good one was having a different employee each month go to lunch with the owner. We had everyone's name on a piece of paper in a box - after an employee returned from lunch with the owner they would draw another name for the next month's lunch date.
I have a lot of examples - let me know if you'd like more!