Kids were put in groups of 3.
Each group received a number line, a clear cup and 20 counters.
Child A secretly put some counters in the cup and showed his friends for about 5 seconds.
Children B and C estimated how many counters were in the cup.
Child A counted the counters and the person with the closest estimate won that round and got to put the counters in the cup for the next round.
The number line was used to figure out which estimate was closest (it came in handy when one estimate was high and one was low).
My kids LOVED this game, and all but one of them asked me if they could play it again tomorrow.
Here's a copy of the number sheet I used (it's from Seomara Range), but you can click on the image for my edited version.
Tomorrow when we play, I'm going to give each child their own set of number lines, so that they can keep track of the two estimates, the actual count, and the difference between each estimate and the actual count.