How to keep celery crisp?
Trim the root end, then set the stalks upright in a tall glass, add a few inches of water, then stash in the fridge.

You'll want to change out the water every couple of days, or can get strange on you.

You can also cut it into shorter lengths, and store it completely submerged in water; this may be necessary to do with part of the stalks anyway, as most people don't have their fridge shelving adjusted to handle the full height of a bunch of celery.