Workouts should be a challenge! If an exercise becomes easy over time (especially a standard one like push-ups) try something different to make them a little more difficult. Resistance training is only effective when strain is placed on the body. We know that for a lot of exercises we can just add weight to increase difficulty, but there are other factors we can add too!
For stability exercises try using exercise balls and BOSU balls to add a twist. Maybe find a light resistance band to add to a standard lift. Or make it fun and challenge your friend to something random like in the video below!
The key is not to let your lifts get mundane!