Try to keep things easy -- do as much prep beforehand as possible.
For my DD's recent b'day, I knew I had to serve food since my in-laws are the type of people who hang around until you feed them a meal (if I timed the party for mid-afternoon, at 6 p.m. the hangers on would start mentioning hunger). So I purchased bag salad from Costco, some salad dressing, the Costco Artisan Bread (yum), and made up 2 large trays of baked pasta (made them the day before and tossed into the oven on the day of the party). Throw in some two liter bottles of soda and a Costco Birthday cake (decent cake and the price can't be beat, $15 for a two layer full sheet cake) and we had plenty of food.
That's my number one rule for entertaining -- make lots of food.
If you've got more than a few kids coming, put together some gift bags -- Target has a decent section for party goodies. If weather is warm, let the kids play outside -- consider a pinata, scavenger hunt, or other organized activity if there are several kids coming.
If it's mostly family, don't worry about doing organized games or anything. Family is usually happy shooting the breeze, eating food, and watching the birthday kid blow out candles and open gifts.
Enjoy!