A varied trek, landscape, vegetation, and culture all change constantly. There are numerous fascinating villages. The chief difficulty is that after several steady days in the Marsyandi valley you reach Manang at 3400 metres, and a couple of days later you are 2000 metres higher on the Thorung La. Be aware of the potential for altitude-related problems.
Things change ! Road-building is going on apace both up the Marsyandi Valley towards Tal (and ultimately, at some future date, to Manang). In the Kali Gandaki valley it's now possible to reach Muktinath by jeep via Pokhara - Beni - Jomson. Buses run from Pokhara to Jomson (and back !).
It's debatable how this road building will affect trekking. It makes it feasible to do the shorter Circuit, for example Besisahar to Jomson, in far fewer days, but presumably trekking and traffic are not such a pleasant mixture. There are numerous lodges along the route, some of them very good.
Day 1: By road to Besisahar
Day 2: Trek to Bhulbule (if you arrive in Besisahar early, or the road has now been pushed further, you could get to Bhulbule on day 1)
Day 3: Trek to Syange
Day 4: Trek to Tal
Day 5: Trek to Latamrang
Day 6: Trek to Bhratang
Day 7: Trek to Bragga (or Manang)
Day 8: Rest / acclimatisation
Day 9: Trek to Lettar
Day 10: Trek to Thorung Phedi
Day 11: Cross Thorung La to Muktinath - the big day, reaching 5400 metres at the summit of the pass
Day 12: Trek to Jomson
Day 13: Trek to Kalopani
Day 14: Trek to Tatopani
Day 15: Trek to Chitre
Day 16: Trek via Ghorepani to Ullieri
Day 17: Trek to Birethanti
Day 18: 30 minute trek to Nyapul, then bus or taxi to Pokhara