Hi Laura,
One of the easiest way to accomplish this without too many actions is to add an action to the OnScreenLoad event to Go to a Tab Link for the default start up screen. Most likely the default start up screen is the 'Welcome' screen, provided you haven't changed it. You then specify which Tab Link you would like to load . The Activities tab link under Sales Center has a default filter that displays all active Activities on or before today.
One point of caution with this solution is that whenever you first load or refresh the Welcome screen it would direct you to the Activities screen. Of course there are more advance means of accomplishing this, let me know if this works for you.