The first idea is use something you already have, weather that's a hat or bag or in my case a flower crown. I simply stuck a nail in the wall (you could use command hooks if you don't want to damage the wall) and I think it adds lots of colour and personality to my room and it also looks cool :)
Another way to spice up your bedroom walls is by using Washi tape, its really simple and you can customize it to whatever design you like. I chose to do arrows and it only took like 5 minutes - trust me it's really easy.
Here's another example of using Washi tape. Here I cut roughly 5 cm strips and stuck them on my wardrobe door in crosses. It was super easy and the good thing is when you get sick of the design you can just peel it off and try something else.