I think the best thing to do is watch what interests him and follow up on that to the full extent of his interest. For some kids in some areas of interest, they're just mildly curious and a quick bit of Internet research or one library book will suffice. But for some kids in certain areas of interest, they'll go nuts with a subject for YEARS, soaking up every bit of information they can find on it.
I'm not the most creative mom in the world (!), but I've found that if we follow my kids' interests, there are some activites that become obvious, even though they probably wouldn't seem like obvious "enrichment activities" to someone else. When DS6 was obsessed with mazes, we checked out every maze book the library had, he drew mazes for me to solve, we read the Greek myth about the labyrinth and the Minotaur, and we visited a giant cornfield maze. When he was 2 and was obsessed with cars, we read the Consumer Reports annual auto issue to him on the potty and he studied the photos; in this way, he wound up memorizing the makes and models of effectively every car on the road! So visits to auto dealerships and even jaunts around parking lots were great fun for him and suited his interests.
Were trips to the parking lot what I would have picked as enrichment? Of course not! But he was in heaven. We followed similar paths with NASCAR (no one else in the family is a fan), Transformers, and now it looks like we're on to foreign language--a change I greatly appreciate! So now I'm researching opportunities for foreign language study and introducing him to foreign words used in English and every bit of foreign language I can remember from my own education.
Follow their interests until the interests peter out, and you'll probably have the best luck (and the most fun!) with them. Just don't invest a lot of money until you're sure it's not one of those mild interests!
