Where in the Fens are you going?
Are you looking at the deepest darkest fens (Chatteris/March/Wisbech/Spalding/Boston/Holbeach etc) or on the edges somewhere?
It's not brilliant, but if you're on the edges near Peterborough and Ramsey on the land side there's a few hills and woods that might be of interest. Not far to Thetford Forest/Brandon from the Swaffham side.
Otherwise, as Mark said...it's pretty much paths on the dykes and coast. It's a spectacularly weird landscape, down to 2m below sea level in places; worth a look.
There is something magnificent and otherworldly about the really remote places, like Gedney Drove End
Just watch out for the bombing runs from RAF Holbeach, mind
All the best,
Pause, take a breath and ask yourself: "What would Rastamouse