Upvote:3
If your main destination is still what it was when you made your visa application, you can make whichever side trips within the Schengen area you want in addition to that main destination, as long as you don't exceed the visa's allowed length of stay.
You do not need any particular invitations or other formalities, but when you initially enter the Schengen area you should be prepared to explain your plans for the entire trip, and document it to the extent it is reasonably documentable. For example, you should have plans for where you will stay and plans for your travel between the various destinations.