The first question would be: Do you want something permanent or not easily moved/concealed? That is are other people going to see it and do you care whether they do?
Imagine if you had a two year old. How would you decorate their bedroom? You can find child themed drapes, bedspread, sheets. We had a clown light for our young son. He still has it I believe.
Child themed sheets and having plushies laying around can give a room a really kiddish vibe, pictures or posters of kid's shows can be good too. If you've got a computer in your room then having a kiddish background or screensaver can help too, sounds random but it can be really good
easily moved/concealed, i want my room to feel like one of those rooms where you press a button and it flips, but its just m converting my room in under 5 minuts to suit my special needs
Maybe find a double-sided bedsheet and brightly colored decorations. You could frame nostalgic artwork that fits in either an "adult" or "childish" room.