I have a 3yo girl, and we went camping for the first time a few weeks ago.
Pitching was fun - she was in charge of pegs and mallet, and telling me which rope to do next (she also came in useful when asked to hold a rope whilst I did the other end.
She's always trying to help, so she tried to pump up her airbed (unsuccessfully), and helped to get the water.
In terms of toys, she was quite happy to sit on the picnic rug with her stuffed toys having a pretend party. The other diversion was to do some colouring and drawing, though her attention span is not that good. She's also good at sitting and playing with her leappad, which can be a little annoying now she knows about the volume control!
All in all, my success factors are to get her involved - if she's doing something, she's going to enjoy it more.
My kids are 7 and 5 and like the other peeps on here they have their jobs to do when we get there, it makes them feel part of it and all grown up too We also have an 'entertainment bag' that lives in our car all the time which has such stuff like travel sized board games, cards, outdoor games and pens and paper etc :) They know they can just nip into the bag and get something out to play with.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.