I second Markfawkner's advice.
You could easily head to the West if you hired a car, and stay in B&B's in around Conamara and County Mayo for a couple of days. It would involve a bit of driving but not a disproportionate amount given that you'll be in the country for four and a half days. I wouldn't bother with this unless you are hiring a car, though.
Otherwise, the Wicklow Mountains would be a good destination. You could also stay overnight in Glendalough or one of the nearby villages or youth hostels. If you want to feel active, the Wicklow Way is a waymarked trail, and while some parts of it are challenging, in good weather there are certainly sections which are quite approachable. For example, you can easily walk up Djouce Mountain from the car park overlooking Lough Tay, or indeed from the other side. That would be a walk of a couple of hours. The Sugarloaf is a much easier and quicker walk but the last part is a steep scramble over loose gravel.