Bekki
I agree with the others. If I was you, I would space the toys in specific areas eg garage and cars in one corner, tunnel in another corner,
trainset/dolls house in another corner etc.
When I did my DS last year I also set the following up outside: tunnel,tent,train set,sandpit,water play.
I played childrens music on the CD player in the background and the kids (age range 2-4 yrs) bopped if they wanted (actually, after a few beers most of the adults were singing to it!)
If you want to do some cheap/fun party bags you some ideas are: bottle of bubbles, crayons, balloon, box of raisens, iced gems, colouring book, stickers etc etc and of course the napkin wrapped piece of cake! There are some companies that will sed you the bags already done - pesonally I love collecting things for the bags - but if you haven't got time, you could do a search on the web to find the companies.
Lots of party food and a cake and you'll be laughing.
Most of all have FUN yourself - that's my motto!
Hope that helps..
Noisy