You can do a nice sea-cliffe walk between Bray and Greystones as a short half-day trip from dublin. The dart (commuter train) stops in both towns, so you can get the train to one of them, walk to the next town 1-2hrs and, grab some food or drink and then head back to dublin. The whole way you will have lovely views out to sea.
I would probably go Greystones to Bray, then you can end by climbing Bray head, which has fantastic views. The station is just up from the beach in Bray, and there is a nice pizza place on the way to the station - platform pizza. Ideally go on a weekday, because it is a popular walk and can get busy on weekends.
Other things in Dublin and around
Dublin
Dublinia - Interesting museum telling the history of Dublin, starting from when it was founded by Vikings. I went for the first time in years, about a month ago and found it really interesting. Its funny the things you forget about history. Dublin went from being a "viking" city, to being an "english" city, and was the centre of British power in Ireland for about 700 years. Its strange because now Dublin is a quintessentially Irish place, but through most of its history that was not the case.
Dublin Castle - Interesting place, get the guided tour, the guides are quite informative and bring the history to life. I would suggest visiting Dublinia first as it will prime you with the knowledge that will make the castle more interesting!
Nearby:
Powerscourt - stately home with beautiful gardens- considered one of the nicest gardens in the world! There is a nice restaurant on site. Travelling with your (new) wife, I think it would be quite romantic.
Glendalough
Monastry and lake, in a really beatiful setting. If you are not renting a car, I would presume that there are regular busses or coaches from Dublin.
