I've tried those dissolving things...they always seem to just make a mess. IMHO, more trouble than they're worth.
If you have a friend or neighbor you trust, have them take care of it for you. Use one of those daily pill containers they sell at local pharmacies to ration out each day's food so there's no question or confusion about how much your fish should be fed.
If you don't, then an auto-feeder may be your only option if you're going to be gone more than a day or two. I've heard good things (albeit, some bad things, too) about the Eheim Air Feeder. My suggestion would be to put it in use for at least a few days, if not longer, before you leave so that you can be sure it's working correctly.