In reply to martynlj:
A few ideas for you:
Around Edinburgh:
East Lothian coast from Longniddry (or Aberlady) to N. Berwick. Not circular, but you can return easily by train/bus. Stick to the coast itself, rather than the John Muir Way, which misses all the best bits.
Plenty of options in the Pentlands - if you want avoid climbing hills there is a good circuit from Harlaw reservoir passing Threipmuir, Loganlee and Glencorse reservoirs and back via Maiden Cleuch.
Fife:
Circuit of Loch Leven
A section of the Fife Coast path, maybe Crail to St. Andrews (not circular, but there is a good bus service).
Perthshire:
Near Dunkeld, assorted options around Loch Ordie and Deuchary Hill
From Pitlochry, the Bealach path from Moulin to Killiecrankie and back along the river (option to do Ben Vrackie on the way).
From Blair Atholl, up Glen Tilt as far as you like, some semi-circular options with paths/tracks on either side of the river.
Glencoe:
The Two Lairigs walk around Buachaille Etive Beag.
Circuit from Kinlochleven to Loch Eilde Mor and Blackwater Reservoir.