What would be the perfect 10 to 14 day trip ?

Train from Oslo to Trondheim (via Lillehammer)
Then on to Alesund, where you could pick up a car.
7 day fjord driving route south to Voss, as described here...
https://norwaytravelguide.no/self-drive-itineraries/a-seven-day-fjord-adventure-
Finish in Bergen.
Flam Railway as you return to Oslo.

Note: I don't have Norway experience, just reading and online research.

Really depends on your interests.
I would concentrate most all of your time to the Fjords but if you are big into mountains, coast, islands, history in cities, museums, etc... Norway has options.
It is a tall country and not many highways so if you want to go far North, even with 10-14 days you are better off flying within the country mid-trip.

You will find Norway less crowded the later you get in August and definitely in early Sept so that is good. I think the waterfalls if that is a big draw for you are less impressive by late August. Some tours/services do stop just after the middle of August which may be bad but also can mean less crowds and busses on the roads. Mid June to Mid August seems to be the peak period in terms of tourists, corresponding with more daylight hours as well. When you will be going days will be longer than back home but only by a few hours most likely.
Weather varies considerably from week to week so harder to predict than most places.

We planned for lots of rain, it ended up not raining one drop once ; that was in late August normally a time when the country gets a decent amount of rain.

Assuming you choose not to visit the northern half of the country.
I would single out your top 4-5 things you want to see/do and map them out and then plan a driving route starting in Oslo. Head North from Oslo up to roughly Andalsnes or Trondheim (again interest dependent those are 2 much different locations) before heading south ending in Bergen ; plenty of spots to stop generally in route to make up your days.

Hotels and food/drink are big expenses, expect them to cost double for similar quality compared to say Italy a place you are familiar with.
Unlike many countries in Europe, Norway has high drop off charges if you are renting a car from one city and returning to another,, something to consider because an ideal plan probably involves flying into Oslo but flying out from elsewhere like Bergen. The dropoff charges also really vary so good to research your options of different towns to rent and drop off the car if you are not planning a R/t pickup/dropoff point.

djp_syd - I will look at the route you gave me tonight. Thanks

mreynolds - I have been reading your report and the route you took sounds great. We do want to concentrate on the Fjords region. Maybe one day in Bergen.

I did 11 days mostly driving in late September 2014. It was nice and quiet as far as crowds go, but lots of things were already closed for the year so I think late August would be a great time to go!

Here's the route I took. Things I loved most: Norsk folk museum in Oslo, stave churches, Maihaugen folk museum in Lillehammer, Trollstigen and the drive south through the fjords, the Setesdal Valley. If (when!) I go again, I'd spend some time in the Innerdalen area and probably stay longer in the fjords - I mostly did 1-2 nights in each place and it wasn't long enough, really. Have a great trip!

Having just done the sort of thing you seem to be talking about, I will offer only these specifics:

1) Lay out a route that includes plenty of cross-fjord ferries. RS calls them mini-cruises and they give you an excellent opportunity to see the country from the water as well as the land, while also giving yourself a break that is valuable for preventing driver fatigue.

2) Consider the National Tourist Routes.

3) The Geiranger fjord is a UNESCO World Heritage site and deserving of it IMO. Take the ferry between Geiranger and Hellesylt, in at least one direction.

4) If your plan includes Route 63 south of Geiranger, do not pass up the road to the viewpoint on Mt Dalsnibba. There's an extra toll, but the view is amazing.

5) Don't take a rental car anywhere near downtown Bergen if you can avoid it.

We just returned from a driving trip (our first) to the Norway fjord area.
The trip started in Bergen. We spent two nights which helped acclimate to the time difference. There is plenty to explore to justify two nights. Hint. Take a cab and walk around Bergen - don't bring your car rental. Fees are huge and why pay car rental to leave your car in parking. Since Bergen is on the coast, always take a rain coat or umbrella even if there is sunshine. It rains a lot.
From Bergen we left early and casually drove north and caught the 12 noon ferry from Gudvangen to Kaupanger where we sailed the famous Naeroyfjord. From there we drove north to Fjaerland where we stayed at the charming, lovely but a little quirky Fjordstove Hotel and restaurant. If you are a senior and have problems with stairs this may not be the place for you but if you can manage, it is a lovely spot to stay, eat and explore the views, take some beautiful drives, and see the glacier, etc. From Fjaerland we drove north to Geiranger where we stayed two nights. There are some beautiful drives, scenic views, shopping, restaurants, to keep you busy here for a few nights. From Geiranger, we began to loop back and drove Route 15 toward Lom and our eventual next stop, Solvorn and the Walaker Hotel where we stayed three nights. The Walaker is a great spot because there are some fabulous day drives, scenic views, things to do, and hikes. The hotel is total charm and lovely. And finally, we headed back to Bergen to spend a night by the airport.
We are seniors and wanted a trip to take photos, be in the countryside, and experience rural Norway. We could stop when we wanted and with this itinerary, we did not rush at all. If you want this kind of vacation, then these are some good ideas for you to consider.