ooooh crikey
OK, OK
How about a craft activity to start with - make a cardboard crown and decorate (with stickers/colours for less mess) or decorate party bags (blank ones available in Wilko's).
Games: musical chairs works well, also msuical statues, also pass the parcel is always good.
You need big games with stuff to do rather than fiddly ones.
A treasure hunt worked well at DD's party - I hid pictures all over the hall and the first one to find all 4 got a prize.
Have you got lots of help?
Bouncy castle or a short-notice entertatiner may be a better idea actually. Also gdg is right on the money with colouring, that kept a few at DD's do amused for the whoel 2 hrs! I gave prizes for the best. Whistle is a very good idea.
Also mummy wrapping game - two or more teams, roll of loo paper, each wraps up an adult (like an egyptian mummy), you judge the winner. Hilarious!