Spending time at the ocean is my idea of heaven. I love collecting shells. And tangling on them afterwards brings back those memories.
I'm at a Zentangle retreat for CZTS in Michigan this weekend and I brought along shells I collected in Florida last winter. We had fun adding our special touch to our shells.
I like to use a Sakura Microperm pen on shells but I know some people successfully use the IDentipen and even the regular Micron pens. Shells are free and they usually aren't handled much so there are several options.