Tikal is definitely a highlight of Guatemala, and if you have the time and energy it's worth making the trip there. Eliot is right, flying there (via Flores) is certainly the easiest way. I'd definitely do that if you can afford it.
However, if you'd rather not make the trek to Peten you can find some other good places around central Guatemala. Quetzaltenango is a large city, but is pretty cool and cosmopolitan. Nice people too. You might also consider making the trip to Coban - a pleasant mountain town surrounded by coffee farms. It's also close to Semuc Champey, which has some of the nicest swimming holes in all of Central America. Check it out: