More freerolls will be nice, specially at more friendly hours for us across the pond, it's quite late here the current daily freeroll of Hedge Poker, and has I've been seeing in other bitcoin poker sites, freerolls seems the way to get an active user base.
The freerolls have been great and they are definitely a good way to get active users.
Other than a referal bonus, the only thing I can think of would probably be too hard to code as I only see them at house games or casinos...
Bad beat table bonus, cracked aces bonus, etc.
Maybe do a weekly freeroll for 100-200 credits for users who have participated in x amount of tournaments.