If you rent from Matt at Hearts and Tears he will be give you all the advice regarding routes you can shake a stick at.
My two cents - the bus ride from Ktm to Pokhara is an experience but there is a shop across the road from the Kathmandu guest house in Thamel which can get you a plane ticket for 55 USD (not the 120 others will try for). It takes half an hour.
Two people and luggage? Rent one of the 500's.
Chitwan? Yep you can ride an Elephant ect but the Bardia National Park (a day further west) is alot less busy, unlike India you are allowed to walk around on foot (and thus not scare off everything within 200 meters with your landrover), and is considered pretty much the best place on earth to see a tiger.
Ask Matt for English John's phone number, I might have it somewhere too. He has some cheap and comfy accommodation in the park itself and some local guides who know where the local wildlife is (ie where the tigers come to drink water at a certain time each day). He's a nice bloke, and put me up for the night when I rode across Nepal.
An evening in the Old Inn in Bandipur (halfway between Ktm and Pokhara) is recommended too.
These are just some of the friends I made along the way on during my time in Nepal.