If you're into hill walking you can't really go wrong in Wicklow. The most obvious route is the Wicklow Way - depending on where you are staying and how you are travelling you could try several different ways of doing it - from shortish day-long walks to doing the full thing over the 3 days and either camping out or staying in B&Bs etc on the way. The area bteween Enniskerry, Roundwood, Glendalough and Glenmalure is one of the best parts, and you also have the option of climbing Lugnaquilla. The Hostels in Glenmalure and Glendalough would be a handy base as would any of the B&Bs in the area. Some nice pubs along there too - as that pint after the long walk is one of the best you'll ever have!
If you've any more specific questions, let me know and I'll try to help out..