How about "Mother May I?", or "Red Light, Green Light", or "Dodge Ball", or a water ballon toss, or the one when you pass an orange using your chin. There's always bobbing for apples! How about a Pinata?

Oh, this is EASY! Spin the Bottle is good, and so is Twister.

You did say the kids are between the ages of 18 and 26, right?

Hehe... been awhile since my last "drive-by", BOBO... Been awhile ;-P

I don't have kids, but I used to use these for the church youth group:
Try relay races like egg on a spoon, balloon between the legs, water in a tablespoon (to fill a cup), 3 legged race, etc. Kids of all ages love these games. Aside from the water balloon toss, egg toss is also amusing, but it depends on how messy you can get.

Kids love wet and/or messy.

Caltechgirl nailed it. Those are good games.

Potato sack races, I use to love those!

Wheelbarrow races (that is where someone holds the legs up of someone else and they then use their arms to race while the other person runs while holding the others legs up).

A scavenger/treasure hunt is always fun too. And you can pair older kids w/ younger kids in teams.

Balloon Popping relays are always popular at my house.

For something different Charades can be really fun (I write things to act out on 3x5 cards -- and it is easy to have age categories for that too... Pictures for the little kids and more complicated scenarios for older kids.)

And we have neighborhood games of kickball and "johnny may cross my ocean."

Good luck with the picnic - I hope you have great weather and lots of fun!

Give them various lengths of PVC pipe, a bag of potatoes, a can of Final Net hairspray, and a book of matches. That should keep them busy for awhile...

I'm with Ogre. Something involving fire.

Just avoid anything involving CLOWNS, okay?

Actually, I was at a kid's party a while ago that featured a karaoke contest. The prizes were Lego sets, action figures, etc. The MC kept things moving fast and the songs brief. The kids loved it and the videos were hilarious!

Bean Bag games work great. We have these at a lot of my family reunions and even the adults get into them. You need 8 bean bags, four in one color and four in a different color. Two platforms that look like 30 degree angled ramps with a 5 inch hole cut into it. Set the boards as far apart as you like. Then you basically play horse-shoes except the object is to get the bean bags into the hole. If you want more information I'll give it to you. But it's cheap and easy.

I always liked Jarts, but the keepers of safety always spoil the fun.

after unloading the vehicles --- hand the older children the list of procedure for the day--- explain that their Competition for the day, with award at the end - is to do everything themselves and the organisers are the judges - one with the highest points wins - you sit back (pen and paper at hand)in relaxed comfortable group and enjoy the day.......with the challenge they should thrive....make certain they understand you will answer no questions on how to do anything....you will be fed..the younger ones organised the older occupied...and just maybe pleasantly surprised at the capabilites of the group