Kai Enshu is one of the resort by Hoshino Resorts located near Lake Hamana with focus on tea.
The scenery itself is not that special, but the location is very accessible (they provide complimentary shuttle bus from Hamamatsu Station, it only takes 45 minutes, perfect for weekend getaway) and if you love tea, you would really appreciate the variety of teas they provide. There was also a tea guessing game held daily. Food is great, as expected from any Hoshino Resort. The box they use to serve the breakfast is also filled with tea leaves.
If you are looking for activities nearby, they have Hamamatsu Flower Park and Zoo within 15 minutes walking distance and Nukumori no Mori within 10 minutes car ride.