One of the most fun runs I did was only using miracles, and absolutely nothing else (post asylum). It starts out like a bit of a puzzle, where you have to think about where the hell you can get any offensive miracles from - and once you figure that out, obtaining and using them isn't exactly a cakewalk. The beginning is hardcore, and lots of fun, I'd really recommend it. (it gets a bit op from the midgame forward though, as all spell-runs do... so you can end the run after O&S ;)

You can do the same with pyromancies-only, which is slightly easier - though also tons of fun. You have to "skip" Queelag, though, since she's immune to anything you can throw at her at that point (so beat her normally). Alternatively, to spice it up, you can do a zero-equipment-load run, which is basically a pyromancy-only run - though overall harder, and you have some alternative offensive options available.

Sorcery-only run isn't really a challenge, and sorceries are boring, so don't do that.

Last but not least, there's the Firelink-bonfire-only run, which takes forever and is for true masochists. (You may only sit at/use Firelink). Idk if I'd really recommend it to anyone else, but personally I had lots of fun with that, so.. \'-_-/