My old, messy fish tank…any suggestions on how to bring it back to life again?

Hello everyone !!

In the picture there’s my first, old and messy aquarium, I know it’s no elegant at all (I was starting with literally no experience at all) , but I want to renovate it and put some decorations in it, I’m looking for some suggestions… I stare at it everyday and I can’t find anything harmonious that could look beautiful inside this tank. Perhaps you could help me. Tank is already cycled, with every parameter in shape.

Thank you all!