Not sure if this topic is buried- or partially submerged- but the slidey stone is mentioned in Janet and Colin Bords "The Secret Country".

I think the Glacial eratic on the East side of Dunsapie hill fort would be quite good for sliding off (if you were inclined to do so) as it is very smooth with a view of Criagmillar castle, the Pentlands and North Berwick Law.

Dunsapie is well worth a look as it has ramparts and some curious rock formations facing west, towards Arthurs seat. Given Ediburghs ancientness i wonder how many lost monuments there are. The boulder near west port by the grass market is intersting as there is also one in the foundations of a building beside it. Also on the anthropomorphic side of things- viewed from Craigmillar Arthurs seat looks like a very fat man laying on his back. Which is interesting as Craigmillar has an interracitve concrete sculpture of Gulliver in one of its parks