Requires a border crossing
The Guatemalan border is just over an hour from Belmopan and Tikal another 90 minutes the other side of the border.
If you’re driving a rental make sure they allow you to take it to Guatemala. If you don’t have permission, drive to the Belize border, park for the day and get a taxi the other side. You should be able to negotiate around $60US for the entire day.
Have the driver take you to Tikal and wait while you visit and then bring you back. If you get the right driver he will escort you around the ruins too. Try and find someone that speaks English unless your Spanish is good enough.
Leaving Belize you will need to pay an exit fee of $30BZ plus $7.50 environmental tax (that money goes to PACT and really is put back into conservation) and apparently there’s a Q10 (around $1.50USD) tourist permit fee going into Guatemala. If you take the car there’s Q40 import permit fee and Q18 for fumigation.
Coming back there’s no entry fee to Belize but the Guatemalans may charge another couple of bucks. Best to pay it, even if it sounds suspicious!