You know whats fun, affordable and takes up little space to store for future parties? Hula hoops. Adults like them, kids like them, and they cost like $3. A cheapo badminton set is fun too - even without a net or anything (kids are good at improvising games).
A party food I love to have is chopped veggies with dip. First of all, you can do it yourself so its affordable and then put it out in batches - left over celery stalks and carrot sticks and pepper strips can easily be used by you after the party (in salads, stir fries, soups, frozen for later use, etc). Its an easy finger food, colorful, and dieters and parents are always happy to have a healthy choice to munch on. A cheese platter again is a crowd pleaser and again, put out in batches so there's plenty, but if you have left overs you can use them later (most cheeses freeze very well).
I like to always make sure to have a bowl of mixed nuts out too - its a nice protein munchie (most party foods I find to be very carb-heavy).
If you are concerned over people socializing (different social groups mingling, etc) a fun thing I've done is to look up some random fun facts (either related to your party - wine facts, or sports related, etc, or to your guests - places you know a few guests have vacationed, or hobby some guests are into) and print them out and mount on a colorful card and tuck them randomly around areas guests will be. If there is a lull in conversation people tend to look around - they will see the card, read it, and IME share the info (like a Snapple fact - don't you always share your Snapple facts?) and voila conversation starter!