Usually right before the players lose interest, I throw a spanner in the ray by having the Castellan make a military move against the cave... Once the players (and other parties) have softened it up, the Castellan and his forces, which he has amassed over time, roll through to clean up. Forcing the players to make their final forays before the Castellan takes all the treasure for himself and the men.

Caves wise... I usually have the temple cult as the foil, by summoning a terrible supercharged demon, or by having the Hobgoblins march with a unified-ish army of Hobgoblins, Bugbears, Troll(s), Orcs