This got me thinking earlier. What's the furthest you can travel in one day and return to your starting destination? As on East Coast you can catch the 07.52 from Aberdeen to Kings Cross arriving at 14.52 which gives you just over an hour to catch some sights before returning on the 16.00 Kings Cross to Aberdeen arriving at 23.12 although even with my free First Class travel it's not a day trip I'd contemplate doing.
