If it is a fixed list of values you can define your own enum type which will automatically get shown as a drop down list containing all the possible values for the list.
If you want the list of values to be variable or you can't use a enum for some other reason, then take a look at Type Converters (http://msdn2.microsoft.com/en-us/library/ayybcxe5.aspx) and methods
GetStandardValues, etc.
Once you have defined your own type converter you will need to tell GAX to use it by specifying it using the "TypeConverter" attribute in your recipe argument declaration in the guidance package xml manifest file.
HTH,
-Victor.