Are you missing 3 footers? If so, circle the hole with 6 balls at three feet (length of your putter), and don't leave until you make all 6 without missing-or set a time limit.
Lag putting-develop feel. To develop feel, line up 20' from a target...a tee in the ground will do. Put another tee inline, 10 foot from you. Hit a ball and approach the far target. Hit the next ball, if it passes the previous ball, it's -1 point. If it's past the first tee, but doesn't pass the next ball, it's +1 point. The goal is to see how many points you can get. If you don't pass the 1st tee, the round is over and record your score...now beat that score.
Playing with a friend? play "horseshoes". Pick a hole. 2 balls each. Sink it, 3 points, within a club length is 1 point, but only the closest get's points. Play to 21.
There are more ideas in the books putting out of your mind by Rotella, and Zen Golf by Parent...both available on audiobook. I listen on the way to the course.