On the beaches I frequent in southern Maine, I occasionally find clumps of Atlantic Dulse (Palmaria palmata) standing in color-contrast to the majority of wave-tossed algae. Much more commonly, I find pieces of Knotted Wrack, Bladder Wrack, Irish Moss, and various Kelps -- none of which have the rich red color of Atlantic Dulse. The blades of this alga can be eaten fresh (when found alive), and dried pieces can be lightly toasted for a fine, salty treat.
Fucus is a genus of brown algae whose members inhabit rocky coastlines throughout much of the world. Perhaps the most well-known type is Bladder Wrack (F. vesiculosus), a species whose branching, flattened fronds have obvious midribs and, often, but not always, feature distinctive paired air bladders (pictured below). All Fucus species are edible and are rich sources of iodine. In his book Seaweeds: Edible, available, & sustainable (2013, p. 85), Ole Mouritsen describes the fresh young growth of Bladder Wrack as "extremely tasty." (For more edibility info, see Green Deane's Bladderwrack profile.) Bladder Wrack commonly shares rocks with Knotted Wrack in the mid-to-upper intertidal zones.
Common Slipper Shells (Crepidula fornicata) can be found along much of the Atlantic coast of the US, and, though small (typically under 2" long), they are recognized easily by the light plate that covers roughly half of the shell's underside. I've read that children (and playful adults) will float these gastropod shells like little boats in tide pools.
Note: A Common Slipper Shell was featured in Quiz #85: Seashore.