Renting a car for the day is OK. Ag Nik to Heraklion / Knossos is not a terribly long drive (unlike from Chania). With 14 hours in port you may have time for a late lunch in Heraklion after Knossos and then an evening in Agios Nikolaos itself which is delightful. There may be time to take the archaeological museum in Heraklion also, although this could be veering towards trying to fit too much in one day. A tendered port call reduces the available shore time.