Nepal, being a long thin country, means there are not to many circular routes, you will end up back tracking, but the road always looks different going the other way!
Check out what some of the tour companies do...
Hearts & Tears Motorcycle Club | Motorcycle Tours | Nepal
even if you can't afford the tours, look at the itineraries.. that gives you a good idea
Kathmandu- a couple of days
One of the safari parks- Bardia is much quieter ie less commercial than Chitwan- a couple of days, (Don't attempt Chincchu to Pokhara by yourself, it is lots of village paths for several days..)
Muktinath trip- epic climb along tough road- 3 to 6 days (perhaps best to do this early in the trip as the weather gets worse during may- torrential downpours in the afternoons, can be quite dangerous)..
One night in Bhandipur, beautiful little town, one night Tansen- same again..
Hope this gives you plenty of ideas to work on..