Landscaping on a grand scale

I mentioned briefly in another thread I plan to change the converted loft, effectively one large room into a grand medieval land.

Buildings aside it's the bits in between that's bothering me, how do others tackle mountains etc. any tips or links? I'm just returning to lego after a 'dark age' so I'm worried i might be biting off more than I can chew.

That said I need a big project to launch myself into and to be ongoing. I figure I can brush up on building skills and change things as I go.

Is it common to build around boxes or duplo as 'fillers'?