Ive been doing squats for two decades......and they ain`t felt routine to me yet !!!!!
Ya know......IMHO..... lil changes here and there can make a mental as well as a muscle difference without changing the exercise but simply changing other things like reps schemes, different rest time in between sets, switching the order of the exercises around, (squats first switch to squats last etc.) rest pause, super set same muscle group etc.
Having said that...... the only exercises I ever really change around are mostly for the smaller muscle groups (bi`s, tri`s, delts, etc.) The big compund lifts are the mainstay and to be quite honest I never really get bored with them. For example, yesterday I did bent over barbell rows but for a change of pace I lowered the weight a lil but at the top of the movement for each rep I held it up against my lower rib cage and squeezed like a mofo
for 3 - 4 seconds. It was the same ol rows but still different.............