E6 sounds kinda awful, (thats the one where people stop advancing at 6th level and just get feats?) Yeah. Yikes. But here's what I do:

Design at the encounter level, not the the combat level.

That is to say: the combat part becomes more interesting when there are secondary goals or conditions:

Special terrain:
Seriously thats the simplest way: the combat takes place on a bridge over lava.. or in a swamp, or in a place that hinders the PCs or  provides an interesting environment. Make sure there is variance and high ground so that player choices make a difference.

Think in 3-D:
Don't just work with flat maps: besides terrain, you can use 3-dimensions to make the combat more interesting: an area where the PCs have to run through an area covered by elevated snipers. Or an area where the battle takes place on shifting platforms or spinning gears can be really cool. Use kids wooden blocks for terrain pieces.

Timers:
Timed tasks make for an interesting twist on the encounter. Monsters that get tougher or change or regenerate after a certain number of rounds, or even conditions on the map that change (so for example, a battlefield that after the first 6 rounds starts to slowly dissolve or break apart, or whatever).

Strategic Tasks:
Not all combat is just combat. You could have an encounter where there are three altars. Touching a stone at each altar attunes it to a certain plane. Once all three altars are attuned, it opens up a teleportation circle that sends the good guys home. The twist is- the bad guys are also trying to seize the altars to summon a demon, so you have a lot of back and forth and outright dodging to try and 'run the board'.

There's plenty of ways to 'game' an encounter like that.