Awesome! You are well on your way. 8)
If it were my tank, I would swap out the substrate (gravel) for something darker - this will make the fish colour up a lot more. I would also get a larger, more dynamic piece of wood, or an additional large piece of wood. It is amazing how much that can create interest in a tank and the fish will like the shelter it provides. I would then attempt to get a bunch of basic live plants - the more the better. Cabomba is easy, as is java fern (which attaches to the wood). To start with, fertilise every day with Flourish Comprehensive (about 1ml per 40L of water - this will keep the plants growing fast and thus help to prevent algae).
If you get more of the same type of danios they will look great and create a lot of interest with their flashy movement. One or two larger fish also make the tank interesting - perhaps a couple of large rainbows (they are peaceful) or dwarf cichlids (can be a bit more challenging).
Just those changes alone will make such a difference, you will be amazed.
Keep us updated about what you choose - it will be great to see how your tank develops.