This is your opportunity to go inside loads of buildings that are otherwise closed to the public. I think it's all free admission. Most things are on Saturday & Sunday but there are a handful on Friday (tomorow).

http://www.ni-environment.gov.uk/events/events-ehod.htm

Last year I went to Stormont Castle and it was pretty impressive, also the nearby Netherleigh which is an old stately home, now civil service building. The tour of the latter was more substantial than I would have expected. The Custom House on the other hand was a disapointment. Campbell College would be one of Belfast's biggest landmarks if it wasnt hidden away.

Freemason Hall at the Cornmarket Bandstand is interesting, and lets face it there's fuck all chance of seeing the inside of it any other day if you're not skilled in the funny handshake business. It can be a struggle fitting all the different sites in one day so I'd recomend giving a wide berth to places that you can see on any given day, ie. Churches & libraries. There are some promising looking schools which while maybe not totally off limits usually they hardly encourage visitors. St. Marys university on the falls certainly looks good from the outside, as does Cliftonville's BRA school.

The Antrim section has 3 places in Newtownabbey, should you be interested.

I approve of viewing the Crombie Building (BRA) it's a lovely building and was a great place to learn in.

I'm working, so not get the chance to make use of the open days unfortunately.