If you have not been close to an active volcano and you are looking for an easy (medium) hike Pacaya is a good choice. You can't go to the top any more and you won't see lava but the views are great and you will walk on old lava flows. Fuego and Santiaguito are very active and you can see Fuego if you climb Acatenango (from Antigua) and you can see Santiaguito by hiking to the Santiaguito viewpoint or if you climb volcano Santa Maria (from Xela). The viewpoint is a moderate hike, the other two are really tough. Make sure you are acclimatized. For Acatenango I recommend the overnight trip. Santa Maria can be done as a day trip (convince your guide to leave as early as possible as clouds roll in quickly). We climbed Acatenango with OX. They want a minimum of 4 people but you have 6. You can also go with Old Town Outfitters. In Xela we climbed Santa Maria with Kaqchikel Tours, a local tour operator. You can also look at Quetzaltrekkers. In Xela there is also the hike to Laguna Chicabal and the scenic hot springs Fuentes Georginas.
You can take a shuttle from the lake to Lanquin but that is a long day. At least 11 hours (the shuttle goes to Antigua first). You can go by bus as well (Pana, Los Encuentros, Chichi, Santa Cruz del Quiche, Uspantan, Coban, Lanquin. That will take a long time too. I like Lanquin, the village and the scenery, and Semuc but again, make sure you can actually visit Semuc Champey (it might be open but the road could be blocked... see earlier posts).
Spend more time in Tikal (stay overnight in the park) than in Flores. If you are going to San Ignacio you can go straight from Tikal without backtracking to Flores. Take any minibus going to Santa Elena but ask to be let off at the crossroads (called El Cruce Ixlu) just south of El Remate. There flag down any bus going to the border. We were picked up by an empty tourist shuttle. It takes an hour or so to the border. Once you have crossed the border you can take a taxi or colectivo to Benque (2 km) and there a bus to San Ignacio.
If you want to go straight to Belize City, take a Fuente del Norte bus from Santa Elena/Flores at 5 am or a Linea Dorada bus at 7 am.