For girls I used to buy creative things, like bead making sets, or baking sets, but can't remember what I used to buy 10 year old boys. Does he have a DS or something simlar & maybe you could buy him an accessory for that. Or maybe a token gift say some 'wicked' socks & money.
I have a 6 year old girl and she loves things like kids make up and jewellery. She also loves crafty kits- you can get a huge range of 'paint your own' kits like mugs ,piggy banks etc.
other than that Polly poket wins hands down in our house-toys r us carry quite a range and the sets very to suit your purse