Last session I did something that may or may not qualify as "interesting" for this thread.
Basically, it was a puzzle beforehand (which worked really well, actually). Then, after the puzzle, they fought a catastrophy dragon (refluffed as an elemental guardian) which every round spat out either a standard or a minion elemental of the same element - at a ratio of standard/minion based on how many failures they got in the puzzle.
The interesting point was that the battles (I used the same format for mutliple rooms, each with its own puzzle beforehand and its own element) were fought in large square rooms with no terrain. Instead, I had the "dragon" sucking in that element from the rest of the room - so, for the fire guardian, the rest of the room was incredibly cold. So, unless the PCs were in the aura of the "dragon" (which grows by 2 each round for 2 rounds, then shrinks back to its original aura 1), they had to make endurance checks or bad things would happen to them.
It worked really well, both tactically and thematically. Plus, it kept all the PCs close to the "dragon" which is where the interesting stuff was happening.
Another situation that was really fun was where I had assassin characters trying to kill a PC who was a noble (an eladrin warlord). Once the party realised every attack was against the PC, they had a blast working to distract the assassins and/or keep the target upright. For this, though, you want to have it being of lesser challenge rating (I think it was an n+1 and was pre-MM3), and even more so if the party lacks healing (mine has a pacifist cleric, the warlord, a paladin and a wizard - so no shortage).
I've also done the "string of encounters" thing - it was worth a try but wasn't as big of a success, as I found it lead to pretty boring battles where the PCs had to use their at wills all the time.