Brook is grateful for many things: his instruments, the fact his Afro remained even after he died, the delicious food Sanji prepares for him and the others every single day, but most of all, brook is grateful he has such a wonderful crew.
Before luffy and the others had come along he had been lonely at sea with nothing to do other than play his violin and wait for the day he either got his shadow back (which didn't seem very likely) or the ship rotted away leaving him to drown the the ocean's dark waters. Fifty long years of solitude and emptiness, but brook knows the wait was worth it.
Brook watches fondly as Usopp tells Chopper some made up story and the little reindeer hangs on to his every word, eyes shining with admiration for the sniper. Zoro is napping not to far away with his three katanas nearby. Sanji is somewhere in the kitchen making dinner and Luffy is trying to steal some food much to the chief's annoyance. Robin and Nami are in the library reading and Franky is downstairs working on a new invention he'll show off after dinner (if he manages to finish it in time that is). It warms Brook's heart (or it would, if he had one. Yohohoho).
Brook is grateful he is part of a crew with a long nosed sniper, a doctor who can fix (almost) any injury and cure any illness, he is grateful for the swordsman with absolutely no sense of direction but always is somehow able to find his way back, the chivalrous cook, the rubber captain who won't let anyone down no matter what, the intelligent archeologist, fiery navigator, and the super cyborg. They are his nakama, his friends and family, and they are his greatest treasure and he loves them all more than anything.
"Dinners ready!" Sanji calls from the kitchen. Brook gets up and starts walking over while humming the tune of his favorite song. There isn't anywhere else he'd rather be than on this ship with these people.