Crossing from Bacalar or Chetumal to Guatemala is very easy. Most of the hostels will put you in contact with a company collecting you in the early morning. The bus driver will help you to get a stamp in the border of Belize, then drive for a few hours to have lunch in Belize city. A second bus will cross the chaotic (but hassle free) border of Guatemala. At 7pm you can have a deserved drink in Flores, it’s not everyday you get to visit three different countries! And you are now near Tikal.
If you feel tired after the trip, Flores is a nice little town to relax for a few days. There are a plenty of hostels, restaurants and good bars with a sunset view. The garden bar of Hostal Los Amigos was my favourite, open to the public and a great place to hang out with other backpackers. If you like to enjoy your morning coffee in the peace of a garden, Cool Beans Cafe is the place for you.
A cheap plan is to cross on the public boat to San Miguel, then walk 30 minutes through a nice forest and get to a beach full of howler monkeys. Bring some food with you and enjoy!
If you want to get away from the gringos (nickname for Americans extended to tourists in general), just cross the bridge to Santa Elena and save some money having dinner where the locals go.
Finally, the main reason to stay in Flores is to visit Tikal. After having seen tons of Mayan ruins in Mexico, I was not very motivated by Tikal. But this place is unique. One hour away from any big town, you will find 16 square kilometers of ancient structures immersed in the jungle and the sounds of the animals. The best part was climbing one of the pyramids to watch the sunset with other travelers. When the magic moment arrived, everybody interrupted their conversations to enjoy the view in silence. Apart from one kid who was too busy dealing with the monsters on his tablet.