There a few beautiful places to visit just outside Dublin if you like walking or scenery. The Wicklow Mountains is great for a day trip. These are really just hills outside south Dublin. They're easily accessible by buses from the town, and they are worth walking through.
At the foot of the mountains is a golf course, and as you gradually climb higher you get better and better views of Dublin.
The walking itself is not too hard going and the surrounding trees and plants are interesting. The Dublin climate is a little wet so everything seems to thrive up here.
There are several paths that you can take through the mountains, all of which seem safe and you're never too far from civilisation.
If you like a day-trip for some reasonable walking, then the Wicklow Mountains will be of real interest to you whilst in Dublin.