If they seem to really enjoying chatting and deal-making with powerful beings, include more monsters that outclass them, but who are willing to deal for the right offer. If they really enjoy outfoxing fiendish traps or turning those traps against their foes, get a few issues of Green Devil Face and sprinkle the contents liberally through your dungeons. If they thrive on Red Harvest/Yojimbo/For a Fistful of Dollars-style cross and double-cross, give them warring factions to play off against each other. If they want romance, toss a handful of potential partners their way and see which ones stick. If they relish overcoming impossible odds, give them adversity. If they’re really about exploring, give them free rein to wander where they will, but tease them with places they can’t reach, knowledge that is forbidden, and secrets that are dangerous. […] Games are about making choices, not rolling dice. 
In addition to that, I feel that the referee is also just another player at the table. Don’t forget the things the referee enjoys.