I'm having a pinata for my ds' 5th birthday next week but I am a little leary of the smashing and grabbing aspects. I'm thinking of converting the pinata to the kind that everyone pulls strings together to open. I'm also thinking of putting the goodies in individual bags so the kids can look for the bag with their name when it opens. I might add some loose stuff like large beads and small craft pom poms for the kids that want to hunt around more. I have a bunch of small toy items, mostly party favor type things, that I've picked up at Target and dollar stores. Other than that, it will just be free play.
Since your dd's party will be at a park, I agree that bubble wands are a great idea. You can make ones out of cheap plastic fly swatters by cutting out some of the swatting end to make bigger holes. You can use deep lids to rubbermaid type containers to hold the bubble solution.