Ah, yeah, the Present Vortex is full of good stuff... Also, there are "Red Ghaj" in there that give out a pretty decent 2300 exp. On the first screen of the magma cavern place, after you make the "bridge", just keep going on the bridge, off the screen, and back to grind the Red Ghaj that appears there. It appears every time, unlike those annoying Wonder Rocks. And besides, you only have to fight, what, 4-5 to get about as much exp as you would if you fought a Wonder Rock? Wonder Rocks just have such a low spawning rate that it's a better idea to just farm Red Ghaj instead.
The Antiqity Vortex isn't bad either. There's a spawning point for Wonder Rocks in the Singing Mountain area, (if you're lucky) and the Snowbeasts there give out lots of exp too... I prefer the Present Vortex, though.