Thanks for all the ideas!
First: I'm definitely going to drop from 5 BTC; I think I'll go all the way down to 0.50 BTC (rather than do 1 or 2). Giving away a percentage of how much the faucet has is an interesting idea, but I want it to be as simple as possible.
Second: I really don't want to make getting coins from the Faucet a whole heavy-weight "register and check your email and yada yada yada."
But I do like the idea of adding an extra hurdle for 'suspicious-looking' behavior. So I'm leaning towards doing some fuzzy browser fingerprinting combined with rate-limiting and, if you look suspicious or the fountain has been giving away a larger-than-usual number of coins, require that you login with your google account before getting any coins. No google account: no coins.
It is hard to create lots of google accounts; they're requiring either phone or SMS account verification these days...