I don't plan as such, well I do, but then usually change my mind when I get there. But what I DO is research the area well so that I can be flexible when I get there, unless I have a specific target in mind.
Recently I did the Grey Corries Ridge, but missed off Stob Ban and Aonach Mor, the first because it was getting late and I wanted to get camp setup before the weather came in, and the latter because it was zero visibility and it involved a there and back to the Bealach, which is boring.
Cycle touring I have an end point in mind each day but meander to get there. The GPS with turn-by-turn navigation switched off is great for that. It just draws a line to the destination and I meander either side of that, usually following the green roads on the Michelin maps.
A fair bit of planning here, but then mostly using public transport and can have non trivial time constraints.... Not that I'm remotely obsessive about following said plans should the weather feel disinclined, some random frill I've added in seem especially absurd on the ground etc etc!
On our last couple of Norway trips I had a plan for the start (where to fly to, how to get to the hills, first hut) which helped to get to a nice place as quickly as possible. From there on there were possibilities (number of days we had vs number of days walking/skiing with variable number of rest days), and I planned ahead so I had the information we needed to be flexible - things like opening times of huts we might end up at, public transport options for potential end points, some strategic B&B details.
I like planning (I have maps and plans for lots of trips we haven't taken, and it is just a form of daydreaming) and tend to overplan, but have learned to appreciate spontaneity - but I like to avoid spending the day in an unpleasant place waiting for a bus.
part plan and part what i feel like and what looks as though it may be worth a look en route and what the weather is doing.
as with martin, when usiing public transport it can dictate how far you stray - especially if there isn't a pub around and 2 hours to wait. day tickets - such as the southern railway downlander offering - can offer immense flexibility over where you start and finish or don't like this bit so get a bus to other parts.
