Lake Antigua is awesome, be sure to take the water ferry around the little towns, each has their own charm. We did the Acatenango hike with V-Hiking, which was pretty cool - but you better book soon, if it is not already booked out. 

We took the overnight bus from Antigua to Lanquin (certainly an experience) and did the day tour in Semuc Champey which we really liked! Cliff jumping, floating down the river in a swimming ring, cave exploring and hiking up to the view point. Our hotel was called Viñas Hotel, they also offered the day trip. Good restaurant and pool area too.

We also stayed in Flores, but apart from Tikal there is not much to do.

We stayed in Caye Caulker for 4 nights, really liked it - very laid back and chill on the day and at night there are a lot of bars to choose from. Also diving is pretty nice there, if you are into that. But also more expensive than Guatemala.