As of April 2018, check boxes can be inserted using Insert, Checkbox from the menus. This tutorial is still relevant however, if you would like to use drop-downs with other types of characters.
If you want a way to choose yes/no in Google Sheets, using a check mark can be a good way to do it. While it is not a built-in function, there is a way to create a check box drop down in Google Sheets. The method is a bit of a work around, but it does end up giving you a drop down with the option of a box with a check mark in it or a blank box. The steps below will get you there quickly.
Note that this video discusses both drop downs and check boxes.
First Step - Data Validation
Click on the menu option and then .
Once the Data validation window pops up, choose List of items from the criteria drop-down box.
Pick from a few different symbols
Copy and paste the two check mark symbols below. These will be the two options that show in the drop down list. If you would like other symbols, they can be found by using option in Google Docs and copy and pasting them back into Sheets. Below are two easy, intuitive symbols.
Following are more options that you can use. Note that you can have the user click a check mark for yes, and just leave the drop down empty for no if you want to just use a check mark not the empty box.
Keep the rest of the options at their defaults and choose . You now have a check box!