Tasks:
//The below query returns a list of Tasks related to Leads SELECT Id, Who.Id, Who.Type FROM Task WHERE Who.Type = 'Lead'
Notes:
//The below query returns a list of Notes where the parent is a specific type, in this case Opportunity SELECT Id, Parent.Id, Parent.Type FROM Note WHERE Parent.Type = 'Opportunity'
Events:
//The below query returns a list of Events related to Account and Opportunity SELECT Id, Subject, What.Type, whatId FROM Event WHERE What.Type IN ('Account', 'Opportunity')