We have a similar situation coming up. My son's 8th birthday is in late Feb. and we have to invite everyone in his class. We decided for that reason to do an outdoor party. (hopefully the weather will cooperate)
But we found the perfect theme to do with the kids that strikes a good balance between organized fun and letting the kids do their own thing - LETTERBOXING!
If you don't know what letterboxing is, it's basically a worldwide game, similar to geocaching without the GPS. You go online and find clues to letterboxes that are hidden in your city (I wouldn't be surprised if there was one already at that park!). A letterbox is a plastic container containing a (usually) handmade stamp. Each letterboxer also has their own personal stamp, stamp pad and notebook. When the letterbox is found, you stamp your stamp into the letterbox notepad (and leave a little note) and then stamp the letterbox stamp into yours. I love it because it includes the treasure hunt, getting out in nature and the handmade art quality of the stamps. Seems like a good fit for Waldorf kids.
There are clues getting you there so it's like a treasure hunt. The clues might be about finding the tree shaped like a hand, going a certain # of paces to the right, looking for the manta ray tree trunk (basically just distinctive features of the area). Or some use compass bearings. You can search for clues ahead of time here
. Be forewarned though, sometimes letterboxes have gone missing, so make sure they're there ahead of time.
I am actually going to hide 6 letterboxes myself since there are none where I want to have his party. We'll divide the kids into two groups and each team will follow the clues to 3 letterboxes. I'll make the clues (and stamps to hide) to follow some sort of fairy tale from box to box - maybe with faeries or gnomes or something.
I'll also make each kid a stamp, which really doesn't require much artistic skill as you can find an image of anything you want online and use tracing paper to get it onto the stamp and just carve it out. Lots of tutorials online. So their party favor to take home will be the stamp made for them, a little notebook (which will have the 3 stamps they collected) and a stamp pad. And then my son will have the notebooks they all stamped and wrote in as a reminder of his party.
Another option if you don't want to make the stamps yourself is to buy inexpensive ones either in the $1 bin at a craft store or online. I bought the notebooks and mini stamp pads at Dharma Trading.
After the letterboxing adventure the kids can just play at the playground and we can eat at the picnic tables.
Sorry this was so long - it really helped *me* to type it all up and get focused.