Trek Stop is a great budget place in San Jose Succotz just west of San Ignacio, near the Xunantunich ruins. The people who run it, a couple from the U.S. and their Belizean partner, are really nice. They have neat little cabins for about US$10 per person, or you can camp. There's a budget restaurant and also here is a small butterfly farm and exhibit.
It's right on the Benque Road (Western Highway), so a bus will drop you or pick you up here. To the border you might flag down a collective taxi (around US$1). There you'll have to go through border formalities (there's a US$10 fee to leave Belize, plus the conservation fee, plus anything the Guat officials hit you for). You can get a Guat bus to near El Remate, where you'll have to change to another bus to Tikal park, or you can take a van or taxi for the hour and a half trip to Tikal.
To do Tikal in a day -- it really merits at least two days with an overnight near the park -- you need to leave early in the morning.
More expensive but easier is to take a day tour with a Belizean company. The folks at Trek Stop may be able to set that up for you.
--Lan Sluder www.belizefirst.com