What is a good idea to do with a 55 gallon empty aquarium?

That's up to you. My brother filled his with stuffed animals because he was tired of caring for it. Ive always wanted one so he turned it over to me and I filled it with water gravel some nice plants and fish. Now everyone loves it. And I get a relaxing way to unwind. As far as what decorations to use, it all depends on your style and tastes. I wanted mine to look natural, so i used river rocks (I think that what they were they are different color browns, blacks, and white stones) with 2 logs ( One that sits up and another that sits down so that it becomes like a tunnel.)decorations and mostly green with some vibrant color plastic plants and a few fake coral I picked up from Walmart. But once again it all comes down to what you want to do with it, and what your tastes are. I suggest looking around at pet stores or specialty fish shops to see what is offered and what catches your fancy. As far as fish go, it is usually best to ask the experts. I did some research but I also asked a guy at petco who was extremely helpful and was able to answer any questions (which believe me when I say were many) I had.