Your tank is small but that doesn't mean you can't do anything with it; you can still grow plants in a small tank, it's appropriate for a betta (do make sure to heat and cycle it though for any fish!) or a few "nano fish." You could also keep shrimp. For rocks, I would recommend going to a local rock yard, you can get them very cheap there, and there won't be as much danger of contamination as you'd have with plucking rocks from outside (that's not to say you can't do the other thing, though). Make sure to pick only inert rocks, that is, (usually metamorphic) rocks that won't leach calcium or metals into your water. If your rocks are going to cause a problem with metal deposits, you'll usually be able to tell by a metallic sheen/rusty color (iron)/bluish color (copper, especially important to avoid) on the outside. Whatever you get, rinse them with hot water and scrub any dirt off, then put them in a bucket of water for a few days and test the parameters of the water (pH for sure, KH, GH, metals if you have those tests) before and after to see if they change, or if the water color changes (in which case you probably shouldn't use them; it's possible to have rocks that change your parameters for a specific effect, but that's something more advanced that I wouldn't try myself or recommend for a newbie). Of course you'll also need some sort of substrate.