BKK-CM is an easy day trip, but the fastest route is also the least interesting.
Alternative routes that can be broken into a couple of days include going via Mae Sot and up along the Burmese border, or via Pitsanulok and Den Chai.
I think you are going to have a hard time finding a place where you can do a one-way rental in Thailand... I'm not aware of any rental shops that offer this service. You could try shipping the bike back to the rental company, which will cost you ~2000-3000 baht, but I can see some potential problems with such a plan...
Actually, riding out of Bangkok is nothing special, and in fact a bit miserable if you've never done it before and don't know how to escape the city. (Motorcycles are not allowed on the toll roads)
Much better to fly to Chiang Mai and rent a bike there imo.
Best of luck!