Been there, done that. My two kids spent three days at Jaguar Inn in the park. The closest lodging is a 30 minute walk from the first ruins and some sites in the park are 30 minutes further from there.
Mayawalk tours from San Ignacio will take you at 8am (two hour drive one way) pay all the fees (border, park,etc) guide you around the park and have you back by 5pm for about $80us. You will only see a few selected sites on a one day trip. Nothing beats watching the sun set from Temple IV or listening to the monkeys first thing in the morning. Mayawalk made a special trip ($80)to pick us up on the third day so we could make the Novelo in Benque and the 4pm Thunderbolt back to AC.
A guide is a good way of understanding things. Good tour guide books would do the same if you are willing.
We stayed at the cheapest park accomodations ($50us a night) and it was fine (see my photos in the Vacation Photos thread). The cafe was OK and had internet access. We did not go to the other cafes but the accomidations were a bit nicer, even a pool.
If you stay in the park, plan on doing several 3-4 hour treks into the ruin sites. Then chilling/eating/naping... My 8 and 12 year olds never complained, I kept it upbeat. You will get tired, wet(in rainy season), hot/wet (dry season is very humid) and sore (walking, walking, walking) from ascending/decending temples.