I'm out of town on my own next week. Whats your opinion on the cheapest and easiest way to fill a tub with something fairly thick?

Watch a few episodes of Big Brother, then sit in it.

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Be sure you have a power drill with a paint-mixer attachment (you can pick up such an attachment for about $1 from your local hardware store), otherwise you're in for a long, tedious mix session. I would also recommend trying to get it a bit thicker than you want so you can add some warm water after you've mixed it up (because it will be cold by then).