Boats
Boating was one of Merry's favorite hobbies. He'd paddle out into the middle of the Brandywine and fish for hours until his skin was tanned and cracked by the sun. Sam, on the other hand, hated boats, and that was one of the reasons he'd distrusted Merry at first, before they really got to know each other.
But as the years passed after the Quest and their curly hair turned grey, Sam began to make short trips to Buckland—"on business", he said, though he wouldn't say what the business was—and he'd quietly ask Merry to show him his way around boats.
Merry suspected his reasons from the very beginning, but he didn't ask questions, and Sam didn't offer information. Nonetheless, Merry showed him how to work the ropes and sails, and he explained their construction and repair and the waterproofing, and he'd keep the thing steady as Sam held on with knuckles quickly turning white.
As the years went by, and Rosie's health slowly worsened, Sam would visit more and more frequently. He got stronger and bolder on his "sea legs", as if he was running out of reasons to stay afraid.
When Rosie passed away, Sam dug a letter out of a drawer and sent it straight away to Buckland.
What that letter said, no one else ever knew. But the younger Brandybucks heard old Meriadoc say, when he'd finished it, "Well, he's off at last. I thought it might be so. I hope he finds him."
Before the funeral ceremonies were over, a letter from Buckland arrived at Bag End that wasn't the typical one of condolence; there was some of that, too, but also a message, and a sealed envelope that he was expressly asked not to open.
When Sam arrived in Valinor, he was carrying a letter in his breast pocket: written by Merry, and addressed to Frodo.