Mindfire
Istar
A trick I picked up back in the Baldur's Gate 2 days: when the gameplay starts feeling samey, do a save and just start killing everything around you with everything you've got. Reload once dead/successful. Nothing breaks up a long series of methodical stealth sections quite like a non-canon murder spree.
That said, with the right powers and tactics you can waltz through a ghost playthrough of Dishonored.
I feel like if I have to artificially break up the monotony of the game, then it's a bad game. Or at least not well designed.