Not that different from today, really. Plutarch says that Crassus owned silver mines, large tracts of land (the most common and important form of wealth, it's safe to say, for Roman aristocrats), and valuable slaves which he could hire out to others. And while it's not really a form of wealth, Crassus loaned his money interest-free, but by doing so was investing in the people he loaned it to, so as to gain favors from them. Lucullus was like Crassus in that he was obscenely rich, but unlike him in that he spent lavishly on luxury, and so he invested in mansions, fine art like paintings and statues, exquisite food and clothing, and so on. But again, the core asset of the typical wealthy Roman was real estate.