DH and I both work FT so we don't mind having full class parties most years for the DC as we don't get much chance for them to have a huge number of play dates throughout the year.
We have run a lot of the party games ourselves in the past on various themes, had a dancing party which was great, science themed.
I'm thinking of a fun fair themed one this year so bouncy castle, popcorn maker, candy floss etc but no actual fun fair rides! Or a sports day one with lots of games?
What have been your hits over the years?