The objects you're using are good, but if you're interested there's some pretty cool stuff at the Dundjinni user creations forum. They're...fancier might be the best word. More elaborate.

Additionally, I've found the bump mapping and drop shadow filters very helpful in creating stairs and raised areas of all kinds. Gradients are also helpful in that area. I'm actually still having a slightly hard time making raised platforms and stairs and whatnot that I'm completely happy with. Trial and error and all that.

I'm a GIMP mapping newbie like you, and I've actually learned a ton from doing RobA's amazingly awesome tutorial on making Regional maps. It's not all applicable to your dungeon mapping endeavors, but what you'll learn about layer masks and overlays are a pretty sweet deal by themselves. I plan on applying a lot of the lessons I learned in his tutorial to my next dungeon.