If you're planning solo, Rishikesh is on top of my list.
if you have time, it's Auli
there are other destinations also like, chopta, chitkul, jibhi, Munsyari which are good for a week's traveling also some of the glaciers which might take some more time and effort but definitely worth it
here are some glaciers in Uttrakhand that I suggest:-
Gangotri
Pindari
Namik
Starting with Dehradun capital city
Nainital
Mussoorie Kausauni
Almora
Auli
Valley of flowers
Chopta
Naukuchiatal
Guptakashi
Pangot are some lovely spots to visit.
The blog is the official website of UK state tourism. It will give you a good insight of the places you'd want to explore. hope that helps.
Https://uttarakhandtourism.gov.in/blog-listing