This afternoon I took a drive up to Indian Valley Bikeworks in Harleysville. At their recommendation I bought a pair of 26" x 1.5" Bontrager H2 semi-slick tires and suitable tubes.
I was able to remove the old rubber with one tire lever, and the new tires and tubes went on easily. They are rated for 60 to 90 PSI. I have them pumped up to about 65 PSI, to provide some cushioning.
In my old cycling days I sometimes took my 1986 Nishiki road bike with slick 700c x 25 tires at 100 PSI down to Forbidden Drive and as long as I was careful, didn't run into problems. I should be OK with the new Bontragers, which are a lot beefier. Of course, I'll need to be more careful than with the knobby MTB tires, but converting the Trek into a pseudo-hybrid with these tires should suit my current riding well.
I'm still tempted to get a spare wheel set and mount the knobbies on it, though.