I also use an estimation jar (It is part of my math center) and instead of having the kids just guess how many items are in the jar I also have them write why/how they came up with that number. I change the size/shape of
the jars I use and I also use different size items. This keeps the kids from coming up with just a wild guess and allows them to really think about their estimation. I've had kids come up with some really great explanations. One time I used a jar with flat sides and one student explained how they counted up the total on one side and multiplied that by 5 (one for each side and extra for
the core of the jar) I give a small prize to the best estimation (they have to come close to the total but also have a good explanation) and the whole class splits what is in the jar.