If you remove Tikal, not saying its not worth it, but its very out of the way with such a limited amount of time and its not a easy place to get to, maybe a future trip visit Tikal from Cancun MX & the Yucatan?
Otherwise, 7 days is a nice amount of time to take in the colonial charm of Antigua, the gorgeous scenery and lakeside villages and towns of Atitlan, and possibly the Xela area, or the Highlands north.
Semuc area is not easy to get too either, and is best incorporated in a trip to Flores/Tikal with some days at Semuc/Copan/Lanquin area....via shuttles....
So you need to decide, but if it was me, and I was in the country for 3 weeks, I would make it easy on yourself, and enjoy Antigua 1-2 nights, Atitlan 2-4 nights, and maybe one other side trip...
Otherwise, to go to lake Atitlan for just 2 nights is not going to be fun and relaxing and enjoyable of at least 3..we spent a week there, and could of stayed another week no problem....
You could fly from GC to Flores, shuttle to Tikal, do 1 nights, shuttle back, and fly back to GC, again, Flores is nice to enjoy a night, and Tikal deserves 2 full days IMO...but thats me.
Good Luck..
Adventure Travel to Colombia, Costa Rica, Nicaragua, Honduras, Guatemala, Mexico, Thailand, Burma, Laos, Cambodia, Vietnam, China, South Africa, Morocco, Turkey, EU, USA National Parks, enjoying culture, cuisine, motorcycling, scuba diving, surfing, sailing, rafting, hiking, fishing, camping, nature, wildlife. Get a Guidebook, and get lost!