For me the key to that is (1) always having something to do, (2) not always doing the same thing over and over again, (3) having a set of flexible tools that I can use to solve a variety of problems, (4) not always worrying about being near-death.
That said, here's my list:
1) Beguiler - tons of spells that can (and usually have to be) used creatively.
2) Druid - good spell list. wildshaping opens up a lot of possibilities. good survivability for a caster that encourages risk-taking.
3) Crusader - you get a semi-random set of powers in combat, forcing you to get creative in order to use them. It works surprisingly well.
-Stuart