How do you add new activity tabs on Leads in Salesforce Lightning? I'd like to be able to add calls/upcoming meetings. This is available in Salesforce Classic.
In Lightning Experience, When you navigated to a Standard Object's related list such as Account you will not find the Open activities and Activity history in the related lists and related quick links. But, when you access the Case or Asset Object, you will be able to see the said related lists.
This is working as designed, the two related list (Open activities and Activity history related lists) will only appear on the Case & Asset object. As part of Salesforce improvements, the Open activities and Activity history' related lists are replaced by the Activity Timeline on objects such as accounts, contacts, contracts, leads, opportunities, and activity-enabled custom objects.
As you want to add Log a call, New task, New event action on the activity tab, You can add it through adding these actions to salesforce Mobile and Lightning experience action by editing the page layouts.
Have a look:-