Between GC and Coban we like the Biotopo del Quetzal. There is a hotel called Ranchitos del Quetzal and you have a good chance of seeing a quetzal there early in the morning. The hotel has a trail as well and the trails in the Biotopo are great (beautiful cloud forest, waterfalls, birds...) so stay a day to explore it all.
Close to the Biotopo there is the Salto de Chilasco : although very rainy and cloudy when we were there we still thought it was an amazing hike and we got to see a large part of the waterfall. The top was shrouded in clouds. Very muddy. Take the rubber boots they have for rent. We went in 2011 and the community had just set up a small welcome center in the village. They called a guide when we arrived. Later we heard there was a land dispute (a farmer demanding money halfway and sometimes not letting people pass). From the latest reviews I gather the dispute has been resolved but they have become quite disorganized. The trail was already in bad shape in 2011, again, extremely muddy and slippery.
Between Semuc/Coban and Flores:
We have spent time in Chisec: it is a small town where we were the only tourists. Not much to do, we walked around for a bit, ate at a comedor, watched a soccer game. But from Chisec you can visit the Bombil Pek caves, just follow the main road north for 2 km. There is a sign. The first time we were there we were expecting to pay for entrance to the caves and for a guide but there was no building or shack or anything. The trail was easy enough to follow, through fields and hills, nice views. After maybe another 2 km we reached the entrance to the cave, we almost missed it, just a hole with stairs leading down into it. Luckily we had our headlamps and as there was still nobody around we descended into the cave and set off exploring. We saw beautiful formations, stalactites and stalagmites, it was a lot of fun and a bit scary as we tried to not get lost. There was supposed to be a small cave with paintings (hard to get into) but we did not find it although I crawled into several ones that looked promising. Afterwards we followed the trail a bit farther to the point where you can rappel into the caves. It looked very steep and not for the faint-hearted but quite an adventure. Now, the next time we were there we were stopped by a farmer who explained to us we needed to get a guide at the "office" in town. Also, we found out it is likely we did not see the Bombil Pek cave but another one (that is why we did not find the paintings). So if you go, ask your hotel where the "office" is and get a guide and see all the caves (and maybe even go rappelling). You can also go tubing.
Another time we left Lanquin on the 5:30 am bus to Pajal (I know you will be driving). What an adventure that turned out to be. The minibus was packed and the ayudante pointed to the roof. Apparently we were riding on top of the bus ! The road was very bumpy but we very much enjoyed the ride as the sunrise was magnificent. At the Pajal cruce we waited for half an hour for a bus to Raxhuja. The scenery was amazing and the bus was packed. It was a dirt road, almost all the way to Sebol. In Sebol we waited for 5 minutes for the bus to Raxhuja. Raxhuja is not what I would call a nice town, just one dusty street. We ended up in hotel El Amigo, probably one of the roughest hotels we stayed in. Strangely enough it had air conditioning, a tv and a shower with cold water but the floor was sticky and overall the place was not very clean.They also had two monkeys on a chain, not something I like to see. Hotel Cancuen looked nicer but we did not see the rooms. We took a pick-up to the Candelaria caves. We went to the Complejo Cultural and Ecoturistico. There is a lodge there as well and a restaurant. For Q40 per person a guide took us to the caves (short walk). The caves were nice, very wide and very high, and we liked the light that seeped from the ceiling into the cave, the river and the blue hummingbird we saw. There are two other entrances to the Candelaria caves along the Raxhuja-Chisec road. We found a decent comedor in Raxhuja and a restaurant that served fish, nice change. We also went to the church for a bit as a crowd had gathered for gospel singing and to listen to a very animated preacher. The next day we took a minibus to Sayaxche. It dropped us off at the ferry which very quickly got us to the other side of the river where we got onto a minibus to Santa Elena/Flores.
Tikal to Rio Dulce: Yaxha of course. When we were in Tikal it was not very busy but it was even less busy in Yaxha. Hardly any people. Lots of howler monkeys and colourful birds. We liked climbing the temples, the views of the lake and jungle. There are viewpoints high up in the trees. Nice mossy trails. A great path down to the lake.
There is also this place in Poptun but I have not been: http://www.fincaixobel.com/