We always have a party for my son's birthday in december and it has to BE somewhere (like a nature center, play area, etc) b/c our house is too small and it is too cold to do anything outside like a park. So this year he got really excited about his half birthday (I guess 4 1/2 is a big deal!) and I finally got to do the party I'd always wanted - a fun, low-stress, low-expense party outside! Loved it!!
If you are having it in a park, you don't need any entertainment; it's a park! We had our get-together in our boring backyard (no jungle gym, no sandbox, etc.) and even that was enough to keep 20 kiddos busy for hours. I set up our little blow-up pool that a few kids used, got out the trucks, balls, frisbees, sidewalk chalk and turned them loose. We did have a pinata from Target b/c my son had always wanted one. We hung it from a tree and the kids thought it was great. The candy inside doubles as a take-home (for the record, none of the kids at this summer gathering knew it was a PARTY, just a fun playdate with a lot of friends. My son knew he was 4 1/2 and did go around telling others, but there was no expectations of birthday stuff).
Instead of cake/cupcakes, we did ice cream sundaes. I got three different flavors of ice cream and my son picked out every topping he could find at the store (we still have some left). Parents scooped for their own kid(s) to make it easier on me and let them control portions and we set up a toppings bar on a table outside for the kids to all create to their hearts' content. Big hit. Paper bowls, plastic spoons, tons of napkins/wipes and it all cleans up in minutes. And if you are worried about entertainment, it certainly took a while for kids to make and eat their masterpieces.
We also do a lot of picnics with friends for summer holidays or camping and some of things we bring along are sack races with pillow cases, plastic horseshoes, bat/ball, soccer ball, bean bag toss and use some of these more than once to create an obstacle course or invent a new game. But really, if your park has a playground, you won't even need all that for this age. My son plays just on the school playground every freakin' day after school for at least 30 minutes with his 4-5 year old friends. When we all go to a park after 1/2 days on fridays we are there 3 hours and they still want more.
So, the really short answer? A park is a great, cheap and way FUN party for this age!
Oh, and if you need to reserve a shelter or grill or something, that might incur a fee. We've been to many parties where the parent just gets there early and puts their things on a random picnic table (not in a shelter) and that's home base for the party.
Have a wonderful time!!