This blog and comments pretty much cover all the bases here.
1. You could just turn it on or off by going to Tools—>Options, expand Projects and Solutions, select General, and then uncheck the box next to Track Active Item in Solution Explorer.
2. You could create a Macro and turn it on or off when you want it. The site has the code for doing the macro
3. You could add the option to the view menu you going to Tools->Customize, View, Track Activity in Solution Explorer, and then moving that to the View Menu
4. The approach I selected. You could bind the action to a keyboard shortcut by going to Tools->Options, Environment, Keyboard, finding View.TrackActivityInSolutionExplorer and clicking in the "Press shortcut Keys" box and then entering your shortcut. I used Alt-T. This way when you want it you can click Alt-T and then you can use Alt-T to turn it off. What this method also does is pretty mimic the activity in step 1.