If you often work with activities in MSCRM, you've probably noticed that there is no way to re-open an activity once you've marked it as completed or closed. This can be frustrating for users because they may want to simply edit the notes after they've closed the activity but there is no out-of-the-box functionality to do this. In almost all CRM implementations that I do, I create workflows to re-open the most used activity types, like phone calls and tasks.
It's a very simple on-demand workflow. Keep in mind that you'll have to create separate workflows for each activity type. Set it up to run "As an on-demand process" and uncheck the boxes under "Start when."
In the steps area, add a "Change Status" step and set it to Open.
You could also use a check condition step to confirm that the activity status prior to updating it but that may not be necessary for everyone.