With it's nautical theme and "prow" pulpit, Sinclair Seaman's Presbyterian Church on Corporation Street is well worth a visit. Open Sat 10am-5pm, Sun 1pm-5pm.
The Grand Opera House have tours (on the hour) from 11am to 4pm on Sunday 13 September.
The recently restored "pink" St Malachy's Church (24 Alfred Street) is open on Saturday between 10.30am and 4.30pm, and Sunday 1.30pm-6pm,. Organised tours at 2pm both days around the magnificent Tudor Gothic church.
Campbell College at the top of the Belmont Road is open on Saturday between 10am and 1pm.
One of the new venues this year is the little-known Antiochian Orthodox Church and Icon Exhibition (on the corner of Cliftonville and the Antrim Road). Open Saturday (10am-8pm with Vespers Service 6pm-7pm) and Sunday (10am-6pm with services at 10am and noon).
The School of Music has recently moved out, but the scene of many a nervous music examination in their old listed building (can you believe it's listed) at 99 Donegall Pass will be open on Saturday 9am-noon. Cancel that, it's not opening!
Stormont Castle and Parliament Buildings (often mistaken for the castle) will both be open. Lunches in Parliament Buildings can be pre-booked by phoning 028 9052 1041. Castle opens Sat/Sun 10am-4pm. Parliament Buildings opens Sat/Sun 10am-3pm. Vehicular access is promised by the Massey avenue entrance.
As well as the buildings there are also treasure hunts and organised tours.
You can pick up the instructions for a child-friendly self-guiding Architectural Treasure Hunt from PLACE (40 Fountain Street) or Belfast Welcome Centre (Donegall Place).
There's a Bus Tour of Belfast's Architectural Heritage leaving PLACE (40 Fountain Street) at 2.30 on Friday 11 September, calling in - amongst others - at Queen's Quarter, St Peter's, Clifton House, St Anne's, the Harbour Commissioners and Stormont. Contact PLACE on 028 9023 2524 for details.
Or what a about an hour-long walking tour looking at Hidden History - The Past Under Belfast's Streets. Leaving the front gates of the City Hall at 11am and 2pm on Sunday 13 September.
Lisburn area (Belfast's bedroom!)
Not too much open in Lisburn, but interesting venues include:
Castle House - the imposing red mansion to the left of
Lisburn Tech (South Eastern Regional College) on Castle Street - was the private residence of Richard Wallace. Open Sat/Sun noon-4pm.
CWS Stained Glass Studio will be running demonstrations between 10am and noon on both Saturday and Sunday. You'll find them in their new premises in Knockmore Industrial Estate (Unit C2, 9 Ferguson Drive).
Unfortunately, there's an error on the NIEA website so the search by county links don't work!