Within the main canon, the only things he seems to have (of any value) are his own spaceship and a wide array of heavily customised weaponry and armour. Presumably maintaining those aren't cheap, but there's no real mention of what he does with his (millions) of credits.
According to "Tales of the Bounty Hunters". The majority of Fett's money is saved toward a comfortable retirement as well as the cost of his prohibitively expensive medical treatment
The target. Kardue’sai’Malloc, the Butcher of Montellian Serat. There
was a five million credit bounty on the Butcher, five million credits
of retirement money.
Boba Fett was not the man he had once been. His right leg, from the
knee down, was artificial. Only constant medical treatment kept him
from developing a cancer; the days he’d spent in the belly of the
Sarlacc had altered his metabolism permanently, had damaged him
genetically to such a degree that he could not have had children had
he wanted them; his cellular structures did not always regenerate the
way they were meant to.
It was not the sort of thing Fett took anyone’s word for; he took several steps backward, rifle still pointed at the group, and glanced slightly to the side. In a holofield at the edge of the control panel, a live link to the Guild Bank showed the current balance in Fett’s numbered account C:4,507,303.
Five million credits, less the Guild’s handling fee of 10%, plus the
seven thousand, three hundred and three credits Fett had had in the
account. Business had been bad, recent years.
The relief that washed over Fett at the sight was the strongest emotion other than anger that he’d felt in at least a decade. He could afford to have a replacement clone for his lower right leg; he could afford the cancer treatments that had been bankrupting him. Fett barely heard himself say, “Take him. He’s yours.”