Welcome to the community @Daniel_Kerman! The equivalent of data validation in Coda would be select lists. Instead of picking a “cell” for where you want your dropdown to appear, in Coda you can insert a select list on the canvas or in a table depending on what your use case is.
To restrict the list of options in the select list, you would adjust the “Selectable Options” when setting up the select list. You can hard-code values in the “Selectable Options,” or make it more dynamic by writing a formula so that the select list only shows the values relevant for your end user. This article explains how you can write a formula for the selectable options in a a select list.