You should buy recently retired items, then wait for a year, then sell them. Retired items are items no longer in production, there'll be less and less of them. I bought some books for 500NP two years ago. Now they sell for like 100.000. But I'm not going back to Neopets so it doesn't matter. You should buy the laboratory map, then zap your animals till they get to a pretty colour, then sell them to another user.

Otherwise, Dubloon Disaster, and Whack a Kass is nice games to earn NP.

Err, what did I just say. I've never played Neopets in my life.