I would strongly suggest that you create a foreign key table to maintain this and use a join query to get the value.
If you can't or don't want to use this approach then you would need to use a case statement
SELECT
'CheckedIn_Status' =
CASE checkedin_flag
WHEN 0 THEN 'Incomplete'
WHEN 1 THEN 'Complete'
...
ELSE 'Unknown Value'
END
FROM
tblactivity
ORDER BY
checkedin_flag;
If you use this approach I would suggest that you make it a function - it would be slower but you would only need to maintain it in one place