Adding programmable task pane aka ActionsPane with VSTO is a piece of cake. All you really need to do is just add controls to the ActionsPane object using the familiar WinForms paradigm. Here is how you would show a button on the task pane in Word.
This code produces less than sofisticated task pane but if you remember to do this you are only "this" close to be the ActionsPane expert . If you need to construct something more fancy you do not write any code at all - you use the WinForms designer. Couple clicks to add new UserControl to your project, then couple drag&drops to design the UserControl and set all the right properties in the Property Browser (especially useful is setting the left and right anchors for the text boxes - then those text boxes can resize with the task pane). Then write this code in ThisDocument_Startup event handler:
if you still have not removed the code that adds the empty button then you should see the UserControl stack below the button. This automatic stacking is useful when you want to conditionally add/remove additional UserControls (e.g. you would like to display a currency converter when user fills in expenses part of the trip report - I know the scenario is lame but just bear with me for a moment [:)]).
Additionally your document just became a SmartDocument. It has an XML Expansion Pack (XEP) attached to it. Check out the Tools->Templates and Add-ins ... dialog. You will see that your document references ActionsPane.xsd schema (which is empty and is there because a schema is required to get whole SmartDocs mechanism get going) and has "Microsoft Actions Pane" XEP as well.
Here is a fun thing to try: delete the "Microsoft Actions Pane" XEP. It is hard to get rid of. It is not the case with other SmartDoc solutions, but this one seems to have an extra protection. Any insights on what is going on here?
P.S. I am making my posts shorter and aimed to deliver a short point and I am still on the fence whether this is the right thing to do. Is this better than cramming vacations and introductions into the same post? Or sacrificing the readability and delivering as much information as possible is gonna be more useful when people look for answers later?