Piano and harp ?
Good question! nice to see, for a change. Harp wasn't ......'instantly' coming to me!
And, of course the piano is actually in the Percussion family, although, yes it has strings.
Because they 'need to be struck', that is why it is a percussion instrument.
If you really mean "stringed" instruments as opposed to "string" instruments in an orchestral setting, then a piano and a harp qualify as non-bowed stringed instruments.<br />
Otherwise a classical guitar, and a lute (such as played in a Vivaldi Lute Concerto in a baroque orchestra), I suppose could be a non-bowed string instrument.
Actually, I sang for ten seasons with the Chicago Symphony Orchestra's Chorus, during the Solti and Giulini years.