...dis be mah home turf! Let's see what possibiliies you have:
Route 1:
Even though Hurtigruten is cancelled, leaving Trondheim by boat is very much an option. Fjord1 (www.fjord1.no) operates a high-speed passenger boat service to Kristiansund (pop 23,000). Based on exports of dried fish to Spain and built on four islands, it's worth a quick hop with the boat across the city strait ("Sundbåten") and a stroll on the docks.
Onwards from Kristiansund, the TIMEkspressen route 11 (www.timekspressen.no) takes you along the E39. You could stay on it all the way to Ålesund (pop 43,000) for a couple of hours in charming art noveou streets and a trip to the lookout at Fjellstua with both alpine and ocean views.
If this doesn't suit you, jump off early at the rather uncharming Moa Terminal for a bus switch. Nor-Way bussekspress (www.nor-way.no) operates the route "Vestlandsbussen om Stryn" which offer some spectacular views especially in the first part before Førde.
Route 1B:
From Kristiansund, instead of taking the beaten track of E39, go the Rv 663 Atlanterhavsveien, a spectacular road development at the edge of the coast (www.atlanterhavsveien.net). Local bus operator Eide Auto (www.eideauto.no - only in Norwegian tho'!) operates the route Kristiansund - Averøy - Eide - Molde (look it up on www.177mr.no - route information) which will take you on this eye-boggling drive. From Molde, catch the TIMEkspressen towards Ålesund or Moa.
Route 1C:
Instead of stopping your TIMEkspressen ride at Moa, stay on it all the way to the end station at college village Volda (pop 8000), and catch a connecting bus to Nordfjordeid. From there, head out to Måløy (pop 3,000) by local bus, and have a quick glance at the memorial of the WWII commando raid there, one of the most famous Allied operations in Norway.
From Måløy, yet another Fjord1 passenger catamaran will take you directly to the heart of Bergen (the Nordfjord - Bergen route). It costs a bit more than the bus, but spacious leg room and generous luggage allowance makes it worthwile, especially as good weather would give a lovely coastline sightseeing experience.
Route 1D:
At Moa or Volda, switch to the "Vestlandsbussen over Eid" route (Nor-Way, again). You'll spare yourself a couple of hours compared to the Stryn route, and will have some nice sights through Stigedalen, though it offers less in terms of scenery than the Stryn route.
Route 1E:
In Ålesund, go on the local Ålesund - Hareid passenger boat route, operated by Tide (www.tide.no). From Hareid, take the bus trip across the sub-ocean Eiksundsambanet road to Volda, where you could hook up with Vestlandsbussen over Norfjordeid.
Route 2:
The Bergen - Trondheim express bus route (www.nor-way.no). The only non-aircraft direct link between the two cities. Since you're travelling southwards, you'll pass most of the sights at night. Bring a book.
Route 3:
Mørelinjen (www.nor-way.no, again) will take you to Surnadal, where you could switch to Dag- og Nattlinjen, from Åndalsnes with direction Oslo. This will take you up Romsdalen valley, with some really amazing alpine sights, like Trollveggen (www.trollveggen.net), the highest vertical in Europe, and an area infamous for basejumper deaths. You don't want to go to Oslo though, so either leave at Otta for the regular Bergen-Trondheim bus, or stroll further down the pleasant Gudbransdalen to 1994 Olympic city Lillehammer, where you can connect with Øst-Vest-Xpressen (nor-way.no again) for a trip including both stunning alpine landscapes at Flåm and the world's longest road tunnel at Lærdal.
These are, I think, the three most likely obvious routes (with variations). More are surely likely if you start exploring the plethora of local routes, most operated by Fjord1. Your 1 and 1/2 day time limit might need some stretching, though....
