The optimization process and recommendations:

1. Avoid Calling Functions With Indexed Columns (query line: 9): When a function is used directly on an indexed column, the database's optimizer won’t be able to use the index. For example, if the column `StartTime` is indexed, the index won’t be used as it’s wrapped with the function `DATE_ADD`. If you can’t find an alternative condition that won’t use a function call, a possible solution is to store the required value in a new indexed column.
2. Use UNION ALL instead of UNION (query line: 47): Always use UNION ALL unless you need to eliminate duplicate records. By using UNION ALL, you'll avoid the expensive distinct operation the database applies when using a UNION clause.

The optimized query:

SELECT
        StartTime AS ID,
        StartTime,
        (SELECT
            GROUP_CONCAT(B.intActivityID) 
        FROM
            tblBookings B 
        WHERE
            B.dtmDateTime = DATE_ADD(STR_TO_DATE(CONCAT('2015-08-21 ', StartTime), '%Y-%m-%d %H:%i'), INTERVAL 1 DAY)) AS `Sat` 
    FROM
        (SELECT
            '10:00' AS StartTime 
        UNION
        SELECT
            '10:30' 
        UNION
        SELECT
            '11:00' 
        UNION
        SELECT
            '11:30' 
        UNION
        SELECT
            '12:00' 
        UNION
        SELECT
            '12:30' 
        UNION
        SELECT
            '13:00' 
        UNION
        SELECT
            '13:30' 
        UNION
        SELECT
            '14:00' 
        UNION
        SELECT
            '14:30' 
        UNION
        SELECT
            '15:00' 
        UNION
        SELECT
            '15:30' 
        UNION
        SELECT
            '16:00'
    ) AS X