M.R.
Love the summer idea, we do this for most summer birthday parties. Get a bucket with the sand digging tools, a noodle, some goggles, a beach ball, a beach towel and some bubbles or side walk chaulk. It is always a hit, you can customize it in color for the gender and get age appropriate items. Blow up the beach ball and put in the noodle (kids LOVE to get a big package, they don't care how much it costs, you can do this for much less than $30!
M.