Hi there,
Hey, this is a trip I've just been looking at!
The best option I've found so far for me is:
Get the bus out to Long Eaton rather than in to Nottingham
Buy a Derbyshire Wayfarer
Buy a Sheffield - York day return.
This works out at about £27 (without any railcard) and gives me the flexibility of using any train. (OK, after 9am...)
There may well be cheaper options by splitting tickets elsewhere but this combination gets me the right price / flexibility balance.
Cheers,
Jason