The BPL's main library in Copley Square currently has an exhibit (in the old, McKim wing) of memorials left by runners and visitors after the Marathon last year. At the end of your walk are several budding trees - and blank tags on which you can write a new message.
At the Old North Church in the North End, you'll find another collection of tags - representing the dog tags of soldiers lost in Afghanistan and Iraq. K.M. Peterson visited the memorial garden recently: