If you just looking to keep things 'interesting' then you should either play against someone on the putting green / practise green or try and beat yourself every time.
I.E. lowest nine holes, most 5 footers in a row, most chip ins or withing tip in range.
When I was practising abit I used to make myself chip in one more ball than last time before I left.
form a compass around a hole (doesn't need to be on a flat green, in fact better if on a sloping green) with balls at N, NE, E, SE, S, SW, W, NW. Now, go around the compass and try and putt all of the balls. If you don't make it, start again. You can also do this with a second, and third, set of balls placed an extra few feet from the hole so you have to go round 2 or 3 times and try and get them all in without missing.