How do I create a blast-proof exp grinder in Minecraft?

A properly constructed XP grinder doesn't need to be made of obsidian, because the mobs will never be able to 'see' you for the purposes of attacking/blowing up.

The trick to constructing such a grinder is to have the mobs' feet on the same level as your head, with this level having the only opening and also having one block of horizontal space between you and the mobs.

Here's a side-on view of such an XP grinder

Mobs drop in here
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|||_ <- You stand here

For a video tutorial, check out this one by Monkeyfarm.

If a creeper is standing in water, then the blast damage is restricted to the water. So you can have water for the mobs to stand in, and if one accidentally blows up then you're not totally screwed with a broken trap, but it will kill the other mobs in the trap.

This is mine: