Weekend went really well. Thanks for tips, everyone.
We drove to Lindhurst and cycled from there to Lymington, ferry over to IoW, then about 8-10km to campsite on the south of the island. We did an indirect route through the forest, seeing lots of horses, and with a bramble stop, so about 25km cycling out. Similar back yesterday, via a different forest route.
Then, on the island we had one day with only a little bit of cycling, but slightly overdid it on the other day. We went to the Owl and Monkey sanctuary near Newport, which ended up being just under 50km all in. Lots of breaks and made a full day of it, but she was getting a bit tired of sitting by the end. Main thing was to forget time and, when she said she wanted to stop, we stopped at the next sensible place.
Camping went well. We were lucky with the weather. Will need to think harder about sleeping bag / bedding for her next time. We were also able to get all three nights at the same site, which saved a lot of work having to move on for the final night.
Final highlight was when we got back to the car, my wife said 'would you like to go in your seat' and she replied 'I want to go on the bike!'
NF/IoW was a great location as she loved the horses, the ferry, beach, etc. Not sure where we could go to match it.