You should try setting up an obsidian farm! It requires at least one bucket, but will go faster the more buckets you have. Consider the following picture:
Surround a 1x7 trough with some safe blocks. It doesn't matter what it's made of, as long as the blocks won't burn away (so avoid wood, cloth, etc). This is the object outlined in green below.
Now you should have a 1x7 depression -- you want to put lava blocks into this. Be careful when emptying your lavabuckets, because if you try to place a lava source block on top of another lava source block you'll empty your bucket, losing the lava. I didn't have lava accessible, so for the picture I used cloth. It is outlined in Red.
After you've got your seven lava blocks, simply place a water block on the rim of trough, indicated by the light blue outlined wood block in the picture. The water will flow over each of the seven lava blocks, turning them into obsidian.
This is known colloquially as "obsidian farming".
It is entirely possible and I built all my grinders this way (before the introduction of XP, which makes it less desirable to kill monsters, and hoppers, which can rescue items from all sorts of death traps). The key is to place the cactus one block above the water and use diagonal flows. This is a picture of the grinder of my main mob trap, but this design is also suitable for installing in one corner of a dungeon wall to grind on a mob spawner (especially if you mine the floor of all mossy and plain cobblestone, leaving a slot perfect for the water flow).
Mobs are pushed against the sand block by the water. Once they randomly jump up and stand on the sand block, they are eventually killed by the cactus. Their drops then usually land in the water below. The water's flow splits evenly around the sand block and rejoins at the other side where the flow goes to the collection point.
Restricting the cactus to be two blocks high as in this image means that the grinder also generates a modest supply of cactus blocks.
Originally, I stood at the point where the flows rejoin to collect items, but a creeper somehow blew up the trap while I was AFK, and I had problems with chickens pushing me out of the collection area, so after that I had the items carried some distance away in an aqueduct (this was before hoppers).
This particular installation is for my main mob grinder, so besides the cactus there is also a fatal drop from above; the cactus here is merely to kill the ones that the water saves from instant death.
You can visit this trap and my other cactus traps at my Minecraft server at mc.switchb.org
; take the Nether route to the “old base”.
Best Answer
First of all, backup your world directory so you can try things over and over again. (Although you should remember that you might spent more time trying to rescue yourself than you'd need to get your stuff again)
Then set the difficulty to peaceful which slows your dying (but doesn't prevent it!). Now if you have some blocks in your inventory on a hotkey, after unpausing immediately look down, switch to the blocks and hold SPACE (jump) and hold right click. If you're lucky this is fast enough to build a tower out of the lava (which is hopefully not too deep). You'll still be burning, which doesn't kill you on peaceful, but if you have a bucket of water put that on the side of your tower to get a small waterfall which also obsidianizes the lava below. Good look!
Alternatively you can use a map tool to change the lava pool you're in into water, but that would really be cheating.