"Not the bowed psaltery anyway, which is a modern instrument, 1960s anyway."

Much too late. There were even Marx Musical Instrument Co. bowed psalteries in the 19th C. I don't know how much earlier, though, and, like you, I am sure they weren't "medieval" in any way.

As for what to play, the early versions of fiddles -- the viols and the rebek were contemporaneous, the recorder and other flutes were not unknown, the jew's harp, of course, is OK, and harps were well-known (though, as with the lute, not for the average peasant or traveling minstrel, due to cost). I think I'd opt for the recorder (or another kind of fipple flute -- I'm not sure it the modern recorder was known in its present form) and a jew's harp (I love them things). If you want to fake it with a guitar-lute, German ones show up on eBay a lot, and aren't so expensive (it depends on the timing, if you need it before you could get an eBay prize from overseas).

Then again, you could look for a hurdy-gurdy or a krumhorn. They may not be the proper era, but they're unusual enough that people will give you the benefit of the doubt just for oddity.

Good luck.

Bob Clayton