Here are some project celebration ideas:
- Take them to a picnic (half a day)
- Take them to the beach (full day including lunch)
- Have a day of scheduled activities: I remember long time ago a project manager once planned a whole day of activities, starting with a breakfast, and then a few hours on the beach, and then lunch, and after lunch it was beer tasting, and after that he took us to a roller coaster, and finally it was dinner. That day I can never ever forget. It was really fun, but it was very costly.
- If you're cheap you can buy some Chips and some Beer and have a small party after 3 PM in the office.
- A lavish celebration idea is to take them for a small retreat in the woods for a couple of days (you should make sure you pay for everything), the only problem with this idea is that some people won't be able to make it (kids, etc...).
- Taking them to the movies is lame, but better than nothing
- Taking them out for a fancy dinner is nice, but easily forgettable
Note that all your celebration ideas should happen on a work day, this means that you must give your team a day off (for free). You should also make it clear that absenteeism is not acceptable, and everyone should be present because this should be considered like a normal work day (some people might say that they would just stay home).
I think the most important thing is to put yourself in their shoes, will they appreciate your plan, will they always remember it?
Whatever you do just make sure you can do something better, or at least similar, for your next project celebration (that's called managing expectations).