It would be almost the same as a follow up. You can store the information you need (the name of the Lead and any associated info) into action results when the checkbox is checked using 'Retrieve Value from Screen' action. Then on the save button, you can insert a record into the activity table to create the task, using the values you saved into the action results. You could even send email reminders, much like our follow ups do. Again I'd make a copy of the activity screen and go through the actions on the save button. It contains most of the actions you need to do a customization like this.
To create a custom Activity Type, you could either insert the new activity type into the ActivityType table (when the task is created), or create them before hand based on the catagories you have. You can get to the Activity Type List screen by clicking the ... next to the ActivityType dropdown, or by viewing the screen from the Customize > Screens tab. If you decide to create the activity types when the task is created, be sure to use Conditional Statements to make sure a duplicate activity type is not created.
- Adam