Well, the big question is how much are you willing to spend? If this little guy is your son then you're going to be looking at the bigger, more expensive options, but he's your little brother or a friend's son, you'll probably be giving yourself a budget.
Whatever relationship you may have with the kid, you need to find out what his interests are. There's no point buying him a new baseball mitt if he spends all his time in front of a computer and hates sports. Conversely, if he is a X-box fanatic, it's hard to know what games he has, or hasn't got, unless you go through them all.
If you're his parent you'll know better than anyone what he's into. Maybe it's time for a new bike or a Nintendo Wii after all. At least if you get him a bike it will give him plenty of exercise, even a Wii is a really interactive gaming console that will keep his heart rate up. If he's into music, how about an iPod? That way you're encouraging his artistic side too.
If you're looking at cheaper options, you could take him out for the day, letting him choose where to go and eat. For a brother, or nephew, it's also a great way to bond, sharing an experience that you'll both remember.
If the present is for someone who isn't family and/or you're not sure what they like, a gift card from a department store is always a safe option. It may not be an inspired choice, but at least it won't disappoint.