get one one of those double-decker bus things, as long as it's the hop on/off kind you can hit anything to see in the city for as long as you want. the drivers/tour guys are helpful..yeah, sounds touristy but is pretty convenient
phoenix park is huge, you can go through it on one of those buses, see the presidents residence, and deer, birds, trees if you like that sort of stuff
also, if the weather's decent you can pop on the train that runs along the bay. it's not a long trip to hit 1-2 of the little towns on the coast, don't remember any name except dun laoghaire.
or you could just spend both days sitting in a pub. there's beer, the people are awesome and no walking around
