I'm thinking of using an old 1993 Trek Antelope 820 Sport Trail to be a "credit card" tourer. That is, lightly loaded, small rear pannier only, used for up to a week at a time. The one I have is in good shape. All parts other than tires and bar grips are stock.
1) Recommended 26" touring tire?
2) It's got flat handlebars -- would one of those trekking bars fit? I am not sure if handlebar diameters from the early 90s are the same as today.
3) The front derailleur, a Shimano Altus C10, is pretty crappy. The rear one is not too bad. Any recommended upgrades for that, or just leave it alone?
Other than obvious things (adding a rack, making sure everything is checked, tightened, and lubed), are there any other recommended upgrades/changes?
Add a rack, get or do a tuneup, and get the bike adjusted to fit you. Go for a couple of longer day rides. Go for a couple of overnight trips.
Now, are there any things bugging you about this bike?
If so, go ahead and address these. But don't start with the idea that a bike from 1993 needs "upgrades" to get you touring. There are hundreds of thousands of marketing dollars floating around the idea that you need to be convinced that your bike is inadequate and you need to buy more things. Almost all of those dollars are wrong.