I fixed up a few problems but there are undoubtedly more. Please let me know what's not working in this thread. There are some extensions that are not supported on UCP (like <tabview>) so we'll have to work around that where we can.
I replaced <activityfeed> with the 5 most recent changes. It doesn't look good yet because the RecentChanges tabs and the abbreviations show up, but there is a bug ticket in to get that fixed.
If you liked the old WikiActivity, it is now available in your bottom toolbar or you can go to Special:WikiActivity in the URL. It takes a while to load right now, but hopefully will improve soon.
As of right now, Message Walls and Comments have not finished being migrated. You'll see a new banner when it it complete.