I think you want some fast growers.... Some stem plants can add some spice to the tank and grow rather quickly. You didn't mention much about your lighting, etc.
But, I would assume its fairly low and you aren't planning to add CO2 anytime soon.
That said, there's plenty of stems that you could add that should be quite at home and grow rather quickly to fill in the tank a bit more.
Water Wisteria
Bacopa Caroliniana
TIger lotus (although not a true 'stem' plant, it does grow fairly quick)
Water sprite (can be floated or planted)
Floating plants
Amazon frogbit
Water sprite
Hornwort