On the Isle of Wight, if you start in Cowes, head down to Freshwater via Yarmouth. Along the military road to Ventnor, then back up towards Newport again via Blackwater and back to Cowes. That works out around 75km.
Lovely views along the military road, and whilst there are a few bumps on route it's not a bad ride. You should be able to do it in 2.5 to 3 hours.
If you're staying on the outer edge of the Isle of Wight you could just do the round the island route which is handily signposted for you. It is slightly longer than you wanted at about 65miles but is a nice ride. Personally I'd avoid as much of the Military Rd as possible (especially between Chale and Hook) as it is just a straightish drag that can have a nasty head wind.
Posts
Lovely views along the military road, and whilst there are a few bumps on route it's not a bad ride. You should be able to do it in 2.5 to 3 hours.