Our teachers arranged out last day for us at 6th form - it was pretty boring. They had a quiz, then they paid for out lunch - which was a meal out of the canteen. We got to sit off in the common room and chat all afternoon (which, tbh is what we all did in free periods anyway, so it wasn't anything different). The sixth form council also arranged a trip to our local zoo for us, and we hired a social club and had a party - that was pretty good
So, yeah, you could:
~Arrange a trip out somewhere,
~Have a party (we all had to pay £3 each for a ticket, and if you didn't have a ticket, you didn't get in. We also invited lower sixth form come, and old students from the school who went a different college),
~Have a 'fun day' (fancy dress, game of twister, cake sale, etc),
~In school, the year 11s could bring in eggs, tomato sauce and flour on the last day. They had the last lesson off, and went into the canteen. They 'dressed' themselves in the food, and when the school kids came out, egged them and poured flour/sauce on them. If you didn't want to join in, you just left. You could do something similar, or have a water fight.