How can I SELECT multiple columns within a CASE WHEN on SQL Server?

The problem is that the CASE statement won’t work in the way you’re trying to use it. You can only use it to switch the value of one field in a query. If I understand what you’re trying to do, you might need this:

SELECT 
   ActivityID,
   FieldName = CASE 
                  WHEN ActivityTypeID <> 2 THEN
                      (Some Aggregate Sub Query)
                  ELSE
                     (Some Aggregate Sub Query with diff result)
               END,
   FieldName2 = CASE
                  WHEN ActivityTypeID <> 2 THEN
                      (Some Aggregate Sub Query)
                  ELSE
                     (Some Aggregate Sub Query with diff result)
               END