yes nxt is pos, but you don't get interest on your holdings. you gather fees by contributing to network security. the process is called forging. similar to mining, when you forge a block you get the fees of all transactions in that block. you need a relatively large amount of nxt to forge blocks regularly. i'm not sure how profitable it is at the moment, but the margin is small! the plus side of this is that nxt has no inflation.

you can also lease your balance to a forging pool, so you don't have to have your wallet open. could make things more profitable. check nxtforum.org or google, i'm not up 2 date on forging pools.