I don't know if I will lay claim to being an expert but I have been, planned trips for others, and am going back in October. We've mostly spent time in the south western part near Antigua, and Lake Atitlan. Most of my adventurous clients in Guatemala are going for the volcano hiking. You can do a single day hike from Antigua on the Volcan Pacaya (there is a guy who cooks pizza at the top from the steam), Acatenago is the most popular and is considered a multi-day hike. Other activities are coffee and chocolate plantation visits but are more leisurely.
Flores in the north is a great add on if timing works, it's next to the jungle of Tikal National Park and there are some great wildlife spotting opportunities.
As far as stays, El Convento or Santo Domingo in Antigua are the two best. Casa Palopo in Lake Atitlan (if you stay in the villa portion they have full view of the volcano on the other side of the lake. In Flores I'd suggest bolontiku or la lancha.
Feel free to reach out if you have more specific questions or if you'd like some help with the trip, though I know you're very well travelled!
Luxury & Adventure Family Travel Agent
Personal Travel Concierge
Europe • Africa • S. America
www.voce-travel.com
Kyle.regitz@fora.travel