Pokhara is a Nepali city 200m away from Kathmandu on the shore of Phewa lake. It is a major tourist spot filled with temples, monasteries and museums. Sikkim near to Siliguri border which faces eastern Himalayas popular for biodiversity, alpine climate and view if Kanchenjunga is a lovely tourist spot. I would suggest Sikkim.
As a person living in Sikkim, I would suggest Sikkim. We're slowly getting over winters here and summer is about to set in. March and April are great months to visit this state and perfect for hikes and treks.