Outside Activities for Wide Range of Ages

I wouldn't plan anything just get some balls , soccer balls, footballs, etc, set um out there and let um do what they want. Just make sure there's someone there to monitor them. I don't like to let my kids play in the front yard unless it's on their bikes on the side walk. Sidewalk chalk is also another good thing but that's more for the younger crowd I don't see an 11 yr old playing with chalk. He/ she may bring a hand held game system to keep them from being bored with all the little kids.
the slip and slide is a good idea but you also need to factor in a bunch of wet kids running around your house.

Maybe do some face paints and have the older kids paint small designs on the little one's faces. A pinata is always fun. OOOH how about tether ball? a ball on a pole, tied w/ a rope! Then the ball wouldn't go into the street. You can have the kids play simon says... Um, ooh, here can be a neat idea! Video record them making an exerecise video! Hey, I can try that w/ my kids! I don't know really anything else, hope some helps.

Water balloons are great, the sprinklers, a bottle of bubbles and various size wands or even one of those bubble guns, if you have a driveway or walk way (side walk chalk), regular balloons (if it isn't a windy day), kites, yo-yo's, card tables and board games like uno, twister (makes a great outdoor game too).
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Almost forgot, musical chairs (our little ones would almost always win because they could slip right under a bigger kid unnoticed.)

kiddie pool and a sprinkler toy, bean bag toss, rent a bounce house...

Can't go wrong with a slip and slide.

They have cool little foam rockets in CVS. They don't go too far,they're only $5. My 5yr old daughter, and 11 year old nephew love them, as do I. So, I'm guessing they're good for all ages. Frisbees should be easier to control than a bouncing ball, water shooters. I think the balloons are a great idea. I've also seen paddles that come with a velcro ball. I believe that was in CVS as well. You could do a little craft table. Any age would like fabric paint, and some items to design. Older girls like making those friendship bracelets. Just shooting some thoughts out here. Lol, hope this helps! Just enjoy the gathering, kids always have their imaginations!

Badminton. You can usually find a cheap set somewhere, or spend more if you want it to last. They can swing like crazy at the little "birdie" but it won't go very far, won't roll down a hill, and the rackets are so light the little ones can handle them. For a small backyard, how about croquet? They can set up the course themselves and practice knocking balls through the wickets.