Almost every places in Nepal has been explored already as it is a small country. I am from Kathmandu but I also haven't visited most places, only few places outside the city I am living. If you are visiting Nepal, you will enjoy trekking in the Himalayas more than anything else. You can go for ABC Trek, Mardi Himal Trek, Gosaikunda Trek. Mardi is pretty easier trek for beginner. I will suggest you visit Pokhara and Mustang.