Tikal is considered by most to be the greatest ancient Mayan site on the planet. So many of the structures are excavated and restored, and the ancient city is so vast.
San Ignacio sits just a few minutes from the Guatemala border and only a few hours' drive from Tikal, which is close enough to lure most travelers who have the time and the money.
There are three ways to do it: on your own, with a shuttle service, or on a guided tour.
If you want to visit Tikal on your own (by far the best option), you'll need a rental car that can cross the border. The only rental company in Belize that permits this currently is Crystal Rental (http://www.crystal-belize.com/). A little Spanish will help, as the border crossing can be confusing. The vehicle must be sprayed with insecticide ($10), and you'll pay around $30 for an importation tax, in addition to your own immigration charges into Guatemala, an exit tax of $17.50 from Belize, and a $5 bridge crossing fee just beyond the border.
Make sure you have a good map, and resist the temptation to turn right into the main area of Melchor de Mencos, the first town in Guatemala. The road to Tikal (currently unpaved at this section) goes straight ahead up the hill, even though it looks like an alley.
I suggest that you try to arrive at Tikal (an hour and a half or more once you cross the border) after 3pm. Your $15 admission ticket will be stamped for the next day, so you get 2 days' admission for the price of one.
The hike into the site takes about 45 minutes at a relaxed pace from the parking lot. Many local guides will offer their (expensive) services, and you'll get more out of your trip with a guide if you can afford one.
Poke around the Great Plaza for a bit, then watch the sunset from the Great Pyramid in the Mundo Perdido (Lost World). Make sure you have a flashlight for the long, dark hike back to the car.
Spend the night in Flores, a 1-hour drive from Tikal. It's an enchanting little village on an island in the middle of Lago Peten Itza. Expect to pay around $10 to $15 for a nice room--the Hospedaje Dona Goya II is recommended.
Spend the next day back in Tikal visiting the outer ruins, the Central Acropolis, and the Bat Palace. Leave early enough to drive back to the border in daylight so you don't hit any people or animals on the road (a real danger)! Some people sleep in the road, and I'm not joking!
The Guatemala border crossing closes at 9pm!
A shuttle service from San Ignacio provides transport to Tikal, then to Flores, back to Tikal the next day, and back to San Ignacio. You'll pay around $75 to $100 for transportation alone.
A fully guided tour to Tikal from San Ignacio can run $300, all expenses included.