Dreamstone
When one Jenny Everywhere's new boyfriend first visited the Restaurant, one of the only patrons who did not appear to be incarnations of Jenny's was a deathly pale man morosely considering “a ruby the size of a baseball”. (PROSE: Second Date)
Behind the scenes
The “pale-faced man”'s description matches Morpheus, the sentient, godlike embodiment of the concept of Dreams, as depicted in Neil Gaiman's highly influential The Sandman comic series. Thus, the large ruby he is holding is presumably the Dreamstone, a magical gem in which Morpheus had transferred a large fraction of his powers, which is central to the first story arc of Gaiman's Sandman comics.
As said arc featured him trying to regain the stone and ended with its destruction, Morpheus's cameo in Second Date, if it slides into continuity with the actual Sandman comics (as opposed to featuring some parallel-universe Morpheus), must take place at some point prior to the actual beginning of The Sandman from Morpheus's perspective, prior to his capture by the magician Roderick Burgess.
