Came across this thread when I was looking for something else. Thought I'd give an update.
Our sunroom is probably about 3/4 of the way done. And I love it! Even more than I thought I would.
All the options I mentioned here earlier I had to make final decisions on in the past few weeks. I have to say, even though this room was a long time coming, I'm kind of glad I had SO LONG to think about it, because it really helped me know what I want.
Funny to see how my final decisions compared to my initial thoughts.
Here we go:
Wood for walls and ceiling: went with tongue and groove fir after trip to lumberyard looking at all options. Fell in love with this.
Stain: tried some semi-transparents but wound up liking solid stains the best. (Thank goodness they sell those little cans to try!) Going with a neutral, light beigey/taupe solid stain by Cabot. We will stain all boards before they go up in a couple of weeks, after electrical and insulation inspections. Right now we're going to try to do all boards, including those in the ceiling, in one tone, with wall boards vertical and ceiling boards horizontal.
Floor: going with meshed flagstone (already in groupings of 6 together) in earthy colors which are a nice transition from gray tile kitchen floor. I had a heck of a time trying to find these (took the roundabout way searching everywhere

) but turned out they were right in my own backyard.
Furnishings and fireplace/fountain: I'd bought our furniture on clearance last fall and stored it. I'd almost forgotten exactly what it was like but we had an opportunity last weekend to try it out in the room along with the glass pub table. (Surprised to see I have the same set as lukenick which I hadn't noticed before! Mine is lighter rattan and the cushions are sage. lol I also picked up a wicker rocking chair.) Turns out both wicker set and table are pretty big, and with so many windows it didn't leave a lot of wall space.
So I was planning to use an electric "wood stove" we had elsewhere (for ambience, the room will be heated with the ability to shut off heat if desired), but realized that wasn't going to work, so we picked up an electric wall fireplace (which can also have heat on or off) which will be the focal point of the room along with a flat screened TV (debated about having one out there, but figure we'll use the room a lot so we might as well go ahead). With those, out went my fountain

I think. But it's ok. I might be able to get a small one to put somewhere, will really have to see once it's done. Was hoping to find a St Francis one (with animals) but could only find a few online. Maybe I'll put it on my Wish List.
So the room will be pretty simply decorated with just seating surfaces and wall electronics. But it will still have an outdoorsy feel. Forgot to mention I also got a dark brown ceiling fan with "leaves" for blades.
Doors: this has been probably the biggest and most difficult decision. We debated keeping the slider currently there between the kitchen and sunroom and adding another slider leading to the outside. But we decided to change the "indoor" slider to french doors (exterior, for when heat is off) so we then moved our slider to the outside (yay to recycling). Unfortunately, that was before I tried the furniture out in the room and the left sided opening was fine. Once the furniture got out there I realized we really needed a right opening slider

which was what DH and the builder wanted to do in the first place which I vetoed

so now we probably need to switch it out with a right opening slider (a better quality one, actually, that the builder is also recycling) so I feel kind of badly about that. But hey, what's a building project without making some kind of mistake like that?

(I did a similar thing on our inlaw addition w/ same builder, who is a great sport. Guess doors and I just have a difficult relationship, lol.)
Stay tuned for the finished room! Hopefully it will be finished in a few weeks.