I wasn't greatly taken with Nantes itself. The chateau is worth a visit, and there are a few attractive older buildings that escaped the ravages of WW2. Overall, it's pleasant enough place, but not that memorable.
If you are prepared to hire a car, there are a lot of very good places to see relatively close to Nantes.
We've stayed in La Baule a few times which is about an hour from Nantes - fab beach resort. The whole area is very pretty and there is plenty to see/do.