What do you mean by one thread per activity though?
well.. i see now that you added the 2 sidebars back... what i mean is the "recent replies" sidebar.. (second box)...i find it much more useful than the first box and i use it to know which thread has had a reply added... but having the same thread listed there more than once is annoying...
for example, as i write this message there are 3 lines for "Site bugs" in that box, one after another...try to make it so that it only displays one line per thread with the latest person to reply... maybe add a small counter for thread posts and/or a date/time column there too to show the timestamp of the latest change... but definitely do not let it show multiple lines for the same thread.
also, try to disable the "recent topics" side box that's above the "recent replies" box and merge its function with the second box (i.e. show a thread in the active threads list even the if it has one single post).. maybe extend a bit the number of lines allocated for this box too...
Keep the newly formed "recent threads with activity" list sorted by the date/time of the most recent activity on that thread, with the most recently active thread at top...