Find the ids of the students who participate in Canoeing and Kayaking.
select t1.stuid from participates_in as t1 join activity as t2 on t2.actid = t2.actid where t2.activity_name = 'Canoeing' intersect select t1.stuid from participates_in as t1 join activity as t2 on t2.actid = t2.actid where t2.activity_name = 'Kayaking'