Google have introduced in-cell dropdown and validation to spreadsheets. This makes it easy to constrain the values of an individual cell to a specific range or list.
How to access what’s new:
1. Enter data into a range of cells. For example, create a list of destinations on your spreadsheet.
2. Select the cell(s) you would like to validate.
3. Under the Tools menu, select Data validation…
4. Change the Criteria to ‘Items from a list.’
5. Click the button next to the ‘Create list from range’ option and select the range of cells you entered data in during Step 1.
6. Click Save and the cell you chose to validate will have a dropdown arrow in it with the data in your cell range as the potential input values. If you want, you can set a cell to allow invalid data.