I was going to say Iceland also! We went this July with D24 (as well as D27, S18 and S19). We did Yellowstone & Tetons with the family after S18 graduated and a European cruise after S19 graduated. Iceland was amazing!! The landscape is sometimes otherworldly. Plus puffins 
If you are on the east coast it is super easy and flights can even be a deal if you are watching for them.
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