We hold a lot of family barbecues and a reunion is similar, just on a much larger scale. I also got picked as the social director for this year's family reunion (we draw straws every year) which we have every July out at Knoesble's Amusement Park in PA so I've been giving it a lot of thought! These are some of the things that have been big hits in the past.
Dedicate a section for the little kids. A wading pool full of water and rubber duckies and a few cups for pouring for toddlers or pre-walkers with a bunch of towels or chairs nearby for the moms to relax in. A sandbox is also a great place for the younger crowd. It's nice to be able to put a ilttle one down and watch them play in one spot, rather than hold them in a big pool or chase them all over the place.
If you've got no shady areas, rent or borrow some shade canopies or tents and chairs so folks can get out of the sun.
For food, buffet style always works best so people can pick throughout the day. We set up a long table full of aluminum foil chafing dishes and sterno burners and keep them full of macaroni and cheese, chicken nuggets, hotdogs, etc. for the kids and lasagna, hamburgers/cheeseburgers, roasted veggies, etc. for the adults. A big pile of rolls, some coldcuts, condiments, salad and dressing, and you'll end up with happy tummies.
We have several coolers full of juice boxes, canned soda, water bottles and one for adult beverages. Plus two "pour your own" coolers full of lemonade and iced tea and plastic cups. I put a sharpie marker next to the cups so people can write their names on them - otherwise you get 40 blue plastic cups sitting around and no one remembers which one is theirs.
You can do bucket races for all ages - way too much fun! Set up two lines of buckets about 30 feet apart from each other, one line is full, the other empty. Teams have to move the water from their full bucket to the empty bucket using anything from a spoon to a sponge to a ladle - whatever is handy or will make it slightly difficult and a little messy. One person fills up the spoon, runs to their empty bucket, dumps it and runs back to give the spoon to the next team member.
Some classic fun:
- 3 legged races
- egg or water balloon toss (maybe with smocks available if people don't want to get messy)
- face painting
- pie eating contest
- tug-o-war
- watermelon seed spitting for distance (boys/men LOVE this...

)
- bean bag toss games - make a big plywood target, put holes in it with different points / bullseyes / faces painted on it and set it up at an angle for people to try and get the bags in.
You can have cheap little dollar store prizes or candy for the game winners - it makes people, esp. the kids, feel a little special.
It's also nice for people to have something to bring home - maybe small fabric tote bags with "xxx Family Reunion 2010" on them and have fabric paint available so the kids or adults can put handprints on them.
Be sure to set up some group shots to get pictures - have the cameraman stand on a ladder or something so he/she will be able to look down and see everyone's face (or if you've got a hill you can assemble folks on that's good, too).
That's all I can think of off the top of my head... although one other thing I'm doing is taking pictures from last year's reunion and blowing up just a piece of them (like someone's nose, or the back of someone's head, etc.) and having a trivia contest to see who can correctly identify the most pictures.
Oh, and don't forget music! Not too loud to be distracting, but just enough to make background noise and give it a party feel.