One of the most impressive Mayan sites in Central America would have to be the ruins at Tikal in Guatemala. Located about an hour across the border, these ruins are definitely worth the detour during your Belizean Adventure. Some may find the prospect of border-crossing in this part of the world to be cause for worry, especially into a developing region that may be less than stable, but there are a few hints that will help you maximize your time and enjoyment and relieve a little anxiety, too.
My first bit of advice: overnight in San Ignacio. You can spend a full day exploring Tikal. Add to this a few hours drive time on the front & back end and it makes for an exhausting day. San Ignacio is only about 30 min or so from the Belize/Guatemala border and is a point of interest in and of itself. Just minutes from one of the more impressive Mayan sites in Belize, Xunatunich, the town hosts one of the largest farmers' markets in Belize, dozens of restaurants, a bustling nightlife and has plenty of choices in accommodations.
I recommend the Midas Resort just outside of town. The have rooms ranging from rustic cabana to hotel-style, and offer tours to Tikal themselves... which brings me to my second nugget of wisdom:
Enlist the services of a tour service on the Belizean side. They will guide you through customs and arrange for transportation to & from the archaeological site. Most tour operators include park entry and customs fees in their prices. Tour operators in Belize go through rigorous education to become certified and, in my experience, are extremely knowledgeable (and proud) about their Mayan roots.
Lastly, try to educate yourself about the site before your visit. Tikal is enormous and will likely take most of the day to fully appreciate. Your guide will dispense volumes of information, so the more you can prime yourself the better.
Ready for your Tikal Adventure? Excellent! Have more questions? Leave them in the comments section below!