Don,

You need only click the cell, click the dropdown button, then click the desired item.  Three
clicks.

This event-fired sub will alternate between "Choice a" and Choice b" with one double-click.

Private Sub Worksheet_BeforeDoubleClick(ByVal Target As Range, Cancel As Boolean)
Const a = "Choice a"
Const b = "Choice b"
If Not Intersect(Target, Range("H1:H4")) Is Nothing Then
 If LCase(Target.Value) = LCase(a) Then
   Target.Value = b
   ElseIf LCase(Target.Value) = LCase(b) Then
   Target.Value = a
   End If
   Cancel = True
 End If
End Sub

Just paste it from here into the sheet module.  Change the range as desired for the cells
applicable.  And the "choices".  You don't need the Data - Validation.