I've heard
Jenny used because of
Guinevere's closeness to
Jennifer. Genny or
Ginny would work as would
Eve /
Evie.
Gwen would definitely work and you wouldn't have to use the
Gwenevere spelling at all!
Very or
Vere would work and
Vera too. I don't think
Vera is in the least old lady-ish.
Any you like? I really like
Jenny as a nn. WDYT?