Originally Posted by diremage
Solution? Cone of cold, blizzard, or other cold spell on the slightly-cooler magma. Then just walk across. Or wall of stone a bridge (which gets hotter every turn until it melts). Let your PC's be creative within the bounds you've set--if they try any method of flying they're likely to get knocked out of the air, and so on.
For bonus points, make multiple isles in the magma, each with a different method of advancing to the next one. Alternately, Wall of Something, which takes damage from the magma each round. And, of course, once they get to the McGuffin at the end, they have to get -back-. And their bridge already fell into the magma.
My problem with this is that it requires the players to have a specific array of spells prepared. If they've got the right spells, they'll waltz on through. But if they don't, then the DM is essentially telling the players, "Oh, you can't do anything. Go home and sleep so the Wizard can prepare the right spells." And if you've got a sorcerer who doesn't know the right spell or spells, you just screwed the party over. Put in a time restraint (The villain is blowing up the city in 24 hours!) then the players are screwed even more.
Instead, I might suggest something less specific, where the spells used could be more ambiguous. If you take the same lava thing, make the lava a huge persistent image (underneath is just actually floor). Take out the glooping lava so the players have a few possibilities right off the bat (Assuming 3.5 here), without coupling spells
-Lava to Stone
The problem with many puzzles such as a combination of pressure plates is that (okay, I'm ripping this off of someone else in the forum. Just read it a few days ago, I think) , without any clues, they just say, we take as long as necessary to try every possibility, in a binary fashion (i.e. If 0 is up, 1 is down, 0001, 0010, 0011, 0100, 0101, etc).
Allucard's/dauphinous' suggestions are things to follow, for sure. Give open-ended sorts of puzzles. The only puzzles which should only have one solution (such as, you must cast Resistance on the smallest spider) should be ones which are optional. Ideas for open-ended puzzles include (but are not limited to) large chasms, waterfalls, shrinking rooms (moving walls/collapsing ceilings/crumbling floors), etc.
Time restraints are also fun as well. Spawn an enemy into the puzzle room every round. Make water steadily flow into the room. Have a bomb (or contingency fireball, if you're not in modern times) that explodes in X rounds.
Just my 2 cents.