you could have a house party? my friend had one which was good...depends how many people you plan to invite, his house got trashed.
You could go out for the day, get the train somewhere?
go out for a meal?
my friend had a party on the beeach that was good dont know if it would be good now like, the weathers a bit rubbish.
have a party in the park? my friend did that and it was good someone brought a guitar and we all took some stuff for a picnic, depends on the weather like.
How about a ride in a limo? I hired a pink limo for my daughter's birthday recently and it was the best money I've ever spent - a great time had by all.