Changing a field from a text to a Dropdown

Hi

Within the new Activity screen in CRM2011, there are fields for Category and Sub Category.

As just open text fields, I think these could be mis-used. However, if there was a simple was to change these into dropdowns, then I think they would be useful.

Does anyone know of a quick / easy way to do this? I'm fairly new to Dynamics, but can figure my way around most things.

Thanks

Rob