Delhi airport to Rishikesh by car is a good option for a hassle free traveling specially for foreign tourists who are traveling to India for the first time. It's a 6 to 7 hours journey.
Note: Try and leave Delhi between 4-5 a.m as driving time increases beyond 6 hours due to morning congestion in towns like Modinagar and Roorkee.
Mussoorie is a hill station and is around 35 kilometres from Dehradun. My experience was really awesome. I and my family really enjoyed this place. Surely your will love the Mussoorie. There are so many tourist spots available in Mussoorie where you can visit. But my favourite tourist destination was Company Garden and Gun Hill after that Kempty Falls.
Nainital boasts a good number of accommodations, with the popular hill station combining the old and the new, the upmarket and the simple across its slopes. Many of the more refined options occupy commanding positions by the lake, or use height to their advantage, gaining extensive, postcard-worthy views up the hill to the northwest. Mall Road remains the place to look for the majority of accommodations in town, with a fair set of hotels, homestays, and guesthouses.