I know the Peak District very well and have visited many places there, yet there are still some places I haven't seen yet; usually it's because it's difficult for me to get there on public transport...or there's an admission fee that I'm reluctant to pay.
Here's my list;
Ashbourne - a pretty market town that I remember being driven through in the car thirty years ago when we were on our way to Alton Towers theme park.
Crich Tram Museum
Chrome Hill and Parkhouse Hill - I've seen so many beautiful photographs and read trip reports from these locations that I really need to go and see them for myself...and walk along the ridge.
Dovedale - I have been there once, and walked on the stepping stones and climbed up Thorpe Cloud, about thirty years ago.
Haddon Hall - It's fairly easy for me to get here on public transport. I'm considering buying a season ticket for 2017 after enjoying numerous visits to Chatsworth House in 2016 as a season ticket holder there.
Middleton Mine - This is a difficult one. Officially there is no public access, but I've read several reports online of people visiting, and it looks amazing. There are twenty six miles of tunnels on three levels, all large enough for full size trucks to be driven in and out of...and lots of abandoned vehicles and equipment have just been left where they were last used. The mine closed about ten years ago. I think the only way I'd get to visit here is on some sort of organised trip though.
Three Shires Head - I've seen some stunning photographs taken here, yet I've a suspicion that they were taken from a low angle to make the location look more spectacular than it actually is...so I might well be very disappointed if I ever get there.