It's sounds like you have most of it already planned. One of the best parties my kids had one a simple one in the back yard. We played stupid games, where they could win a prize, and ate cake and ice cream. IMO keep it simple.
Blow up a few beach balls and let them play volley ball or see how long they can keep it up in the air.
If it's hot, do a water baloon toss. Set up the sprinkler and let them run through it.
You can do a simple scavenger hunt by handing out a list of items for the kids to find. Or you can do a treasure hunt type, where you start with a clue like "if you go to the well, it will have something to tell" (we have a wishing well in the front yard), they find the next clue there, and at the end they find a prize. The prize can be the bunch of balloons and each child can take one home.
There's no need to waste money on lots of decorations, and you really don't need a theme. A few balloons work for decorations, and you can just buy generic plates and a colorful table cloth at the dollar store.
Just my opinion, but I never give big gifts for a birthday, just something small. Basically the party is the gift, from which the child receives more gifts.