Please be aware that Semuc is experiencing some problems. The locals have been blocking the road and entrance. Semuc is open again but security is not what it used to be.
Read this: http://www.prensalibre.com/guatemala/alta-verapaz/sitio-turistico-semuc-champey-seria-entregado-por-manifestantes-este-martes
And this:
http://www.tripadvisor.ca/ShowTopic-g292002-i1599-k8945638-Semuc_Champey_advice-Guatemala.html
You can easily take a shuttle from Antigua to Lanquin. Once there you can stay in the village or closer to Semuc. You can visit Semuc on your own but if you book a tour (once you are there) to visit Semuc you will be with lots of other people. I am not sure if tours are possible right now. Try to find out what the situation is before you go. Tour operators and hotels in Lanquin/Semuc should be able to tell you whether tours are possible or only individual visits are allowed.
This what we used to do (before the protests started): we stayed in Lanquin. If you do too check out the bat caves at dusk (take a flashlight, explore the caves, sit in the cave at the entrance and wait for the bats to fly out). The next morning take a pick-up truck to Semuc. You can climb to the viewpoint, see where the river disappears under the limestone, swim in the pools, go from pool to pool and do the walking trail next to the river. At the entrance the locals will be cooking, great for lunch. Kids will be selling chocolate. Try it. It is different than what we are used to but I like it. You could go to the Kan'Ba caves on your own as well. Walk out of Semuc, cross the bridge and turn left. They will just add you to a group. They can take you tubing as well. The caves are very adventurous. Some would say dangerous. You swim holding a candle, jump into pools, go down a waterfall, you climb and slide... Our friends enjoyed it but one hurt her toe badly. If you go on a tour it usually includes the caves. One full day for Semuc is enough but as it takes time to get there lots of people like to stay a second day to play in the river, go for a walk, rent a tube, relax...
As for Pacaya. All tour agencies in Antigua offer Pacaya tours. OX even has an overnight Pacaya trip.