I'm probably repeating previous posters but here are my tips:-
I always tell tourists to avoid Temple Bar like the plague and instead stick to the Grafton Street/Wicklow Street/Nassau Street areas (and assorted side streets). It's a really nice part of Dublin city centre. Grafton Street and Wicklow Street have lots of nice shops. Grafton Street is a long, winding street. There are usually street entertainers dotted along the street. At the top of Grafton Street you'll find St. Stephen's Green - a small city park with a lake and ducks.
Nassau Street has the Kilkenny Design Shop which has some gorgeous (but expensive) souvenirs. There's a cafe upstairs. You can also access Trinity College (the Book of Kells) from Nassau Street. If you walk right down to the end of Nassau Street (away from Grafton Street), it becomes Clare Street and you can visit the National Gallery of Ireland.
I had visitors recently and we did a tour of Guinness Brewery. The Brewery is about a 20 - 30 minute walk from the city centre. We got the Luas (tram) from the city centre to Heuston Station, but it's still a bit of a walk from there. And the tour itself involves climbing something like seven floors! We were exhausted by the end of the tour and hailed a taxi back to the city centre. There were lots of taxis dropping off tourists for the tour, so we'd no problem hailing one outside.
We also visited EPIC - The Irish Emigration Museum. I've been planning to pay a visit for years now but like so many of these things, you only do them when you have visitors. Anyway, we were all very impressed with it. We spent three hours there and never noticed the time passing. There are a few interactive areas. If you want, you can also visit the Jeanie Johnston Famine Ship (it's in the same area). We were planning to do both, but we ran out of time. EPIC is located in the CHQ and you'll find lots of cafes and noodle bars etc. there.
O'Connell Street (the main street) was a bustling street when I was a child. There were two cinemas and a big department store (Clerys). Couples used to meet under Clerys clock and buy their engagement rings at McDowells Jewellers. Sadly, the street has really declined over the years. One of the cinemas closed down, as did Clerys (in controversial circumstances). The General Post Office (GPO) and Easons (bookshop/stationery shop) are the only draws now. There's a lot of poverty and anti-social behaviour in the north inner city and it culminated in riots in November. I'd avoid O'Connell Street, especially at night.
If you like museums and art galleries, I can give you more recommendations.
For daytrips I'd recommend
Newgrange - passage tomb - very hard to get tickets though.
Glendalough - round tower/monastic settlement in the Wicklow mountains. Amazing scenery and lots of walks
Bray - seaside town - walk on the prom, climb Bray Head, buy fish and chips, have an ice cream.
Howth - seaside town - admire the yachts, get your photo taken at the lighthouse, eat some fab seafood in the restaurants.
If you fancy a show, Riverdance is on at present, in the Gaiety Theatre. Alternatively, you can see Wicked at the Bord Gais Energy Theatre. I'm not sure if you'll get tickets at this late stage, but you could try.
Enjoy your trip and don't forget to come back and tell us how you got on.