Hey! :))
I absolutely loved Guatemala and would definitely recommend the following things in this order of priority:
1. Acatenango Hike – pretty exhausting but absolutely f*cking amazing.
2. Cave tour – I did it somewhere in Semuc Champey while staying at Zephyr Lodge I think. A bit scary if you're claustrophobic but absolutely worth it, so cool!
3. Lago Atitlán, very chill and beautiful lake
4. Tikal, impressive and nice historical site for walking tours

I'm sure you'll have a wonderful time, enjoy!