I'm thinking some pink and blue gravel for a nice contrast, and a few really cool plastic plants. Maybe a castle and a diver chest that bubbles too, for good aeration. Get a bunch of the low grass plastic plants and put them in a line in the front, then put some taller plastic leafy ones in the back. I'd personally put some betas, goldfish, neons and puffers in there. Maybe like 3 or 4 of each for a nice diversity.
If you're even considering putting plastic plants in the tank, PLEASE do not even think about the word "saltwater"... Erase it from your memory. You'll kill more things in 5 minutes than you have in your entire life.
If you want a freshwater tank, take it seriously and get some nice substrate and live plants. Do your research. I'm sure any reasonable person would choose a nice, realistic planted tank over a gravel-filled tank with plastic plants and guppies.