I just bought a birthday present for a six year old boy (who's also smart and athletic). It was a tough decision because I knew he already had every kind of ball and net available!
We bought him some walkie talkies, an outdoor exploration kit, and a bug vacuum. He LOVES it! It's a great way for him to use his brain while running around outside. We bought everything at Target (on a great sale) but I'm posting links to Amazon in case it's easier for you to shop online.