The tank doesn't evaporate that much, at least from what I've observed, since I've had it. I figured, since we're only out of town for 5 nights, if I leave the tank full, the people watching should only have to add one gallon a few days later and then it should be fine until we get back. They will be by everyday for the cats and to feed the fish, but I don't think they'll need to add water everyday.
I've got a ReefKeeper lite coming next week to mess around with to control everything. I don't know that I'll have enough time (and $$) to set up an ATO before we leave unfortunately.
For those of you without an ATO, how are you handling the evaporation?