There's not much advise in here except beer & food related, what else is there to do during the day?
Depends what floats your boat -
Firstly - get on the Dublin tour bus:
http://www.dublinsightseeing.ie/citytour.aspx
It will take you most of the touristy places in about an hour and a half. Sit on it for the full trip and make a note of the places you want to see more of. Then you can hop on and off. The drivers narrate the journey and are usually great craic.
Usual tourist fare - Guinness Factory/Storehouse, Jameson Distillery, Molly Malone, the GPO, Temple Bar, Dublin Zoo (in Phoenix Park)
Scenery and "quiet joys" - Howth, Powerscourt, Powerscourt waterfall (beautiful picnic area), St Stephens Green (Park), Phoenix Park, Glendalough Heritage site (picnics, walks, area of beauty etc.)
Shopping - Grafton Street, O'Connell Street, Henry Street - quite similar to UK shopping though. Arnotts is a large dept store on Henry Street and Brown Thomas is the Irish version of Harrods - on Grafton Street.
Most of these places are on the bus tour route except for Howth and Powerscourt which you can get coaches or public transport to. Howth is more of a half day visit, and Glendalough/Powerscourt can probably be done together in a day as part of a tour package.