The Animal Pit requires some set up, but adds a lot to an encounter. It is best for low level characters who do not yet have access to teleport or flight.
In this encounter, one or two party members have been captured, stripped of their items, and thrown into a 30 foot deep animal pit. The animal pit is a 25 by 25 foot square, and it has a heavy grate on top that prevents anybody from climbing in or out. The grate is locked in place, though a DC 15 disable device check will unlock it, after which a DC 15 strength check will move it. The grate also provides partial cover to anybody on the other side.
The animal in the pit should be difficult for one or two party members to take on, its CR equal to the party member's level.
The rest of the party is at ground level, fighting whomever threw their buddy in the pit. One of the enemies up here has the key to the grate. During the fight, the heroes can try to unlock and lift the grate, assist their buddy in the animal pit, steal the keys and open the grate, or kill the person who has the keys. The rest of the party should do this before the hero in the animal pit succumbs to the monster in the pit.
Simple, easy to run, and makes for some nice drama!