Assuming Odoo 11 CE:

Activity types are in CRM/Configuration/Activity Types

To schedule an activity in an opportunity, click on "Schedule Activity" in the chatter. There are no more fields "Next Action" and "Next Action Date" in the lead/opportunity form because this was replaced by the activity scheduling in the chatter which is not only more convenient to use, but also allows for more than 1 activity on 1 record, and is more generic (you can specify scheduled activities on any object).

If you want the list of all the activities, the menu doesn't exist in standard but it's very easy to create it. Go to Settings/Technical/Database Structure/Models, then find the model 'mail.activity' and open it, then click on 'Create a menu' at the very bottom of its form view, and specify where you want to have this menu. After that, refresh and navigate to the menu you created.

Regarding dashboards, I suggest you create the dashboard you'd like thanks to the 'add to dashboard' feature in any search view and you'll get it in the Dashboard app. You can also have your dashboard as your homepage of Odoo by specifying the 'Home Action' in the user form (in the Preferences tab).

Note: Make sure to enable the developer mode if you don't see certain menus.