I use only natural things in my tank, gravel, pebbles, wood and live plants. Chunks of bog wood will look very nice in any tank!
Just be sure that it doesn't have any sharp edges that the goldfish can get hurt on.
Personally, I think a bare-bottom tank is incredibly unsightly. So is one that doesn't have enough gravel to cover the bottom. Gravel only lets your goldfish behave in their natural way, which is: Dig for food in the substrate. It also holds beneficial bacteria and you will need a lot of that keeping goldfish!! I have a 2 inch layer in my tank, it's a perfect depth to keep the plants in place, and it's easy to clean with a gravel vacuum.
You don't need a heater, even if your apartment would get a bit cooler in the winter, they're fine with that! It just raises the possibility of them breeding when the warmer temperatures come
When it comes to a filter, choose one that has lots of space for filter media! And it has to pump 10 times the tank volume. So if you'll get a 30 gallon, get a filter that pumps 300gph. You can also combine the filters, so you have one that pumps 20gph and one that pumps 10! As long as you get enough filtration
Plants, go for plants with strong leaves or the goldies will tear them to shreds and eat them in a day. I have Java Fern and various kinds of Anubias in my tank, they're really beautiful and hardy plants.