SELECT
late.*
FROM activities AS late
LEFT JOIN activities AS early
ON late.VisitorId=early.VisitorId
AND late.CreatedAt>early.CreatedAt
AND late.CreatedAt<=DATE_ADD(early.CreatedAt, INTERVAL +30 MINUTE)
WHERE early.Id IS NULL
-- Maybe: AND late.VisitorId='26924c19-3cd1-411e-a771-5ebd6806fb27'