A cool thing about mountain bikes of that era is they take the same stem size as road bikes, so you can put drop bars or mustache handlebars on them, either of which you might more comfy than straight bars for all-day riding. Add narrow-ish slicks and you've got yourself a nice touring bike. That's what I've done with an old, old Giant Boulder. The frame isn't anything too special, but it is cromoly steel and it works.