I can't add anything additional that other people haven't already said, but i think it all comes down to ...
1) look at the environment for the encounter, what can be added that will add a dynamic aspect to the location: traps, hazards, obstacles, dynamic terrain features
2) any additional creatures you can add to the mix, like minions. They don't need to be there at round 1, but can come a couple rounds later -- guards from the other room, or maybe the solo creature summons them (undead crawling out from beneath the ground, demons being teleported in by the solo creature, etc).
Item 1 is hard to say more about without knowing the type of environment.. ie. in a dungeon stuff like flame traps, or on a mountain rock slides, and so on.
Item 2 is hard to say anything thematically-appropriate without knowing the type of creature that is at center stage (i.e. a lich summoning undead would fit, or a dragon calling kobold minion guards from the next room, or a tarrasque summoning in devils, etc).