What do you recommend to see/do for a 1/2 day in Linz?
We were in Linz three full days last October. It is a city that shows its Tyrolean culture. There is a very nice shopping district, a couple old churches, and some great food. The Linz torte, of course, has to be tried, and there are innumerable variations. We found a farmer's market on a Wednesday or Thursday, and had sausage sandwiches with homemade apple wine. We liked Linz very much. There were not so many English speaking tourists as in Vienna where they were leaking out of every side street. I bought a nice tyrolean hat and a tyrolean cardigan for a good price. Let me know if I can help further.
Agree w/ the posts above. I actually went to Linz mainly to see Mauthausen (I was on the way from Vienna to Salzburg, and as you'll see from a map, it's located around 1/2 way btw the 2), but ended up staying 1 night. Touring Mauthausen, like Dachau or other Concentration Camp memorials/museums, is a sad, yet very important, meaningful and educational experience. And as the poster above pointed out, Linz has some pretty sections. I mainly spent a late afternoon just wandering around, after going to Mauthausen in the morning. Linz certainly does not seem to be on most tourists' "Must-See" lists (!); I, too, encountered far fewer English speakers than in, say, Vienna or Salzburg, and don't think I would give it more than 1 night; but that said, I ended up being pleasantly surprised.
Since I am a huge pez collector, the Pez factory was amazing to see in Linz.
If you have transportation, you might like St. Florian's Abbey.