One way to accomplish this is with Groupings. You need to enable Groupings at the site level http://docs.moodle.org/en/Groupings.
Then you can make some activities available only to a particular Grouping. So you would move people who have paid into the Paid Grouping and they could see more activities.
This won't happen automatically on payment, you will have to add them manually.
Another method would be to make a separate course for guest access and then another course which has paypal enrollment.