Not going to Peten this time. I have a couple of weeks, and rely on chicken buses for transportation. One time I went to Tacana and San Marcos, because the next bus went there. Liked them just fine. So, not looking for the top tourist trip, but an idea of some place to visit for a day or 2.

Below are some random thoughts...just my two cents. If you can provide a little more info I am sure others much knowledgeable than me would have a lot to add.
I have never been to Tacana or San Marcos. If you are interested in San Simon, there is one in San Andres Itzapa about an hour outside of Antigua. If you are into smaller cities, you might like Chimaltenango and Escuintla. Neither has much for tourists, but I found them very interesting places to spend a few hours walking around. I know Chimaltenango has motels for 40q and up, but I have not stayed overnight there. On the outside, some of them look pretty decent. Chimaltenango even has a real movie theater (not fake ones like Antigua).
I have heard a lot of good things about Esquipulas, which I was told by Guatemalans is a big tourist spot for Catholic latinos but not gringos from the US or Europe. I am not sure how long it takes to get down there. If you want to save turtles, you could go to the Pacific Coast but I am told it is a pain to try to get there by chicken bus (never did it myself).
You may just want to show up and start taking buses to different random destinations. That's not for everyone, but it sounds like it worked for you before. Sometimes the ride is a lot more fun than the destination. Some other places I found interesting that you can get to from the Antigua terminal were Santa Maria de Jesus (you have to take a microbus for this one), Alotenango, San Miguel Milpas Altas, and San Antonio Aguas Calientes. Especially with the first three, though, you may get bored after an hour or two. It all depends on what you are in to. I actually stayed a long time at the hotel in San Antonio Aguas Calientes. I didn't spend much time in the Mercado de Artesanias, which is all 99% of the tourists visit there. I found it a very interesting town to creep around (creep in a good way) to see what was going on and how local Guatemalans really live. They make a lot of wooden coffins there.
If you like spending a large portion of your day on the chicken bus, you could even go all the way out to Quetzaltenango, maybe stay a few hours or overnight, and then come back. Except for the downtown area by Central American Park, I didn't find Quetzaltenango very touristy.
Another place I found very interesting that I don't hear mentioned that much is Pastores. It is a couple towns over from Antigua (not much past Jocotenango). I personally walked in about fifty different shops one day where they make boots and other shoes by hand. If you decide to have a pair custom made, though, it is probably going to take at least a week (depending where you go). Some of the stores had a lot of pairs in stock, though, for people who don't want to wait. Because of all the Chinese-made fake Guatemalan products I have seen in other parts of Guatemala, beforehand I was very skeptical of Pastores. However, for the most part, it seems like the real deal. Often times when walking into a shop, I would have to tip toe around so that I would not step on a big piece of cowhide leather that the shopkeeper was in the middle of cutting up. By the way, as far as Jocotenango, there is not much exciting in the main part of the town itself. However, there are a couple coffee fincas, if that is your thing.
I quite like San Juan la Laguna. There are tons of little Keqchi communities along the Rio Dulce that you could spend weeks getting to know. San Andres Semetabaj is a lovely little town with a very ancient archaeological site right in the middle of it. The site usually isn't open but it's worth asking. And finally I agree that Santa Maria de Jesus is worth a visit.
Anywhere south of the Mexico border and north of the Honduran one.
I'd mention my fave spot on the lake, but that would be a bit counterproductive since I don't want anybody else going there. Sorry. San Juan la Laguna is pretty wonderful, though seeing half of the shoreline underwater makes me nostalgiac for the old days.
My other favorite spot is any Mayan town where there's a festival going on. Not gonna be everyone's cup of tea though.

¨If you are interested in San Simon, there is one in San Andres Itzapa¨
San Simon is my amigo. Last year I built a fire of some cookies and red candles and such. I asked for a novia. No waiting, next thing you know not one but two beautiful girls are tugging on my shirt. I am a believer.
There was a parade in San Andres Itzapa last year. Very nice. Does anyone know when the next parade is up there?
In Chimaltenango 40q is for a small room, 60q gets you a tv and enough room to stand next to the bed, 80q is for a nice big room with with new tiles. The 25q hotel did not want to rent to me, I did not ask why.
thanks I will look at this post more carefully in the evening