hi, I might be too late if your party's in the morning, but how about loads of raw veggies (carrot, cucumber, celery etc sticks) with some store bought dips (I can recommend Tesco's caramelised onion humous and salsa - but serve them in your own dishes as though you made them yourself ) Also for the veggies - pizza - again, you can throw on a few extra toppings of your own to a plain cheese pizza, maybe some olives, cherry tomatoes, extra mozarella cheese, roasted red peppers from a jar.
If you want a REALLY easy chilli recipe, this one was given to me by an American friend - fry up 1 lb mince, 1 onion, couple of cloves of garlic (the garlic's optional), then add 1 tin chopped tomates, 1 tin Baked Beans, 1 tin drained kidney beans, pinch sugar, and ground cumin/dried coriander/chilli powder or chili flakes to taste. Simmer for half an hour and you're good to go! Might not be authenic chili, but, hey, it tastes good and v.v. cheap and easy to make!
If you're making rice salad, boil your rice in veg or chicken stock instead of just water, it tastes much better. I'd suggest buying a bottle of store bought salad dressing to toss your rice and veg in.
Agree with MamaG, you can't go wrong with stuff like garlic bread, cocktail sausages etc.
For pud the easiest thing in the world is to buy meringue nests from the supermarket, whip up a big bowl of fresh cream, have a big dish of strawberries / raspberries/ whatever fruit you have, and leave people to assemble themselves. For kids, (and adults too ), different flavours of ice-cream, cones/wafers, sauces, nuts, mini marshmallows, chocolate chips, 100s and 1000s - let them make them up themselves, you'll be a legend!