Depends what you're into..
You have to go for a stroll in Phoenix Park if it's not bucketting all weekend; it's "the biggest urban park in the world" to quote my DH ; there's lots to see in there; even a really good zoo. And an obelisk
Stephen's Green's nice too for a quick stroll. Also Merrion Square
The Museum of Modern Art (IMMA) is pretty groovy, there's a few more museums on Merrion Square too.. The National Gallery..
The Guinness Brewery do a guided tour, ending with a pint in the bar which has a fabulous view..
There's Christchurch Cathedral, Dublin Castle, and St Patrick's Cathedral
Grafton Street is good for street performances; I've seen some brilliant stuff.
..wrote this out ages ago but got distracted!!!