Welcome to the forum, you finally made it
It's good that you haven't actually got your tank yet, at least we can try and guide you in the right direction, rather than picking up the pieces, so to speak.
Why don't you want sand or gravel? It really depends on what you want out of a fish tank. I can tell you that it will cost you a lot of money to use marbles. Some people want a tank to look authentic, others aren't bothered (you know what I'm talking about, huge castles or sunken ships in their tank) I think an Oscar would prefer gravel or sand over marbles. Some nice rock, or some bog wood always looks nice. There really is an endless amount of things you can do in a fish tank.