It's defiantly worth doing an overnight trip. Hotels in Flores are pretty inexpensive, and it's such a cool town. When I stayed there a few years ago I had an OK hotel for around $10 usd a night with wifi and hot water. The lake is very cool, and you can take canoe rides around the island. There are some nice restaurants located on the lake with great views too.
Going to Tikal for the sunrise is pretty popular, so if you do an overnight and plan it correctly you can see Tikal at daybreak.