I would avoid clams just because a 55gal is really not wide enough for a full-sized clam, especially with rockwork.
As far as making your tank shine, once you get it going, the corals will grow out and start covering your rocks, and you can have a neat variety of fish in a 55. Even though a lot of what we get in this hobby are baby fish, they do grow into decent sized fish that will look proportionally correct in a 55 (i.e. a 3-6 inch adult fish will look like it belongs in a 55 gallon and it will look fairly large in the tank).