As the admin of the account, you have the ability to see all data even if it is marked as private. With this ability, there are a number of reports, filtering, and widgets that you can utilize to make sure that everyone is performing to the best of their abilities.
If you want to see the number of notes, calls, etc that one of your user logs, you could use the User Activity Report. This article will help you setup and use the report.
If you would like to see everything that has happened on the account, the User History report is what you need. This article will help you setup and use the report.
You can create multiple lists and widgets that you can filter to see what each user is doing within contacts, companies, tasks, and deals.
One thing you are unable to see is the last login date of your users. The support team can supply the dates for you.