Guppies can breed like wildfire so you may have to find alternative accommodation for the offspring in case you become inundated with fry.
You might like to try a small group of tetras such as Cardinals, Neons or Glowlights. All species are fairly easy to keep so long as their needs are met. However the tank isn't particularly large so try to avoid anything which grows any larger than 5-7cm/2-3".