Hi, in 7/8 days you can actually see a lot! I would suggest that from Siliguri, you head to Kalimpong, then Pelling, Ravangla, Yuksom, & finally Darjeeling :)
Hi, if you are planning of self-driving Roadtrip from Siliguri then, you need to be aware of necessary permit for traveling to Sikkim. However, there are plethora of destination in the North Bengal to explore while on a self driving Roadtrip. I along with my friends concluded 9 days of Roadtrip covering entire North Bengal, while touching upon Bhutan border, Sikkim-WB border and also Assam-WB border. As I'm not sure of your requirements, it is kind of dicey to comment anything. If you wish to get a detailed itinerary along with any other information, you can DM me on Tripoto. Happy traveling & cheers 🙂