I know we don't always think of egg cartons as being a math manipulative, but they actually can be a great resource (as long as they are clean and no yolks are showing evidence...eeewww)!
I got the idea at a conference I attended for first and second grade teachers in Missouri a couple years ago. I recently used my egg cartons with my Kindergarteners for predicting where numbers were within a number line and to work with the adding on strategy.
First, to assemble and create these egg cartons takes a little prep time. The ones shown below are using only numbers 1-6, so I cut the second row of egg holders off and then cut them up individually to be the "lids" for the numbers. I then wrote the numbers with a Sharpie on the bottom of the egg carton. I also use these with the whole egg carton, using numbers 1-12. In this case, you need another egg carton to cut the holders individually for the lids. You would still number them the same on the bottom, just going to 12 instead of 6.
With my Kindergarteners, we started off with finding where the 3 was. (Created discussion because some littles did not know where 3 was without turning over all the lids.)
Don't mind this sweet littles lunch on her sleeve :)
Then I asked them to add on 1. Students should have turned over the next lid to find the 4.
I tried to take them to the river on this and turn it back into the addition problem, (3 + 1 = 4), which some of my sweeties got and some will need to keep working.
The next example they started with finding where the 1 was located. Then they were asked to add on 3.
This little chicka started at 1 and visually added 3 and was getting ready to turn over the 4.
While the fella next to her decided to start at 1 and turn over the 3 he added so he could see the numbers.
This activity worked great as a visual tool for adding. You could also label the egg carton by counting by 5's and label the lids 1, 2, 3, 4, etc. to help with telling time to the five minute interval.
Hope you can find this useful! Start stocking up on egg cartons now :)