Here is a lovely little city break to Berlin using some good value flights from Belfast. It's in the middle of September so hopefully there will still be one good weather before Autumn/Winter which allows you longer hours to explore Given its only a short break it's ideal to be centrally located I've picked a great hotel called the Arcadia. It's ideal located near the station and within walking distance of many attractions. I'm not going to lie, there are cheaper places to stay in Berlin, such as hostels and apartments a little further out but I think this is good value for what you are getting, a centrally located hotel, indoor pool and I've looked at a few different reviews and it comes highly rated.
Two return flights from Belfast to Berlin £39.96 book @ Ryanair.
Book 2 nights at the Arcadia Hotel in Berlin £133.50 book at Venere using code in voucher section of HUKD.