It depends on the size of the tank. If the tank is too small it is hard to add rocks to it without making the tank look crowded. Basalt rocks which are a type of lava work well as does granite. Both of these types work well because they do not break down in the aquarium water. Before you put a rock into the tank test it in a bucket to make sure it does not dissolve in water. You could then add plastic plants placed around the rocks to give a nice centerpiece to the aquarium. Real plants are eaten by goldfish so plastic is always better.

If you get a fast growing plant like anachris you won't have a problem with the goldies if they choose to eat them. There are a lot of plants that goldies won't munch on at all. I have java fern, java moss, red ludwiga and have never had a problem with goldies eating them(maybe because I feed my piggies too well). The only thing they do destroy are the moss balls but not to eat, just to play with:)

I perfer the real plants with my fancies because they are safer then getting poked with the plastic ones. I do have some silk ones in my QT tank and they are pretty soft. Find some low light, low care plants and it is pretty easy. Anachris is nice because it gets really long, kind of like seaweed and my goldies love to play and even sleep in it.