All sorts of stuff! You haven't seen some of the weird things that humans carry around? And they've got stuff on them pretty much all the time.

[Back at the little cove where Nagisa hoardes all his junk, he's got a lot of trinkets stored that he's pilfered from the beach before. He's never stolen things from people at least, but you'd be hard pressed to believe otherwise given the sheer amount he has. From broken cellphones to lost jewelry or wrist-watches, flip-flops and sunglasses with the lenses popped out, and even just pieces of reflective trash or glass that Nagisa found interesting.]

If we're lucky, we might find some cool looking shells and rocks too!

[...he's a seal, he can't help but have fun with rocks.]