A lot of them. The use of electronics is certainly more interesting, but if the use of electronics is not possible you can also invent a lot. Some that first came to mind:
1. Anagrams;
2. You can think of some hint of a certain book and in the book to hide a riddle or a clue;
3. To hang a portrait, which would look somewhere and look at a riddle or key;
4. Invisible text. Take the white paper, the ink pen. Instead of ink, fill the pen with lemon juice or milk. And write a riddle. After drying, the text becomes invisible and can be read only if you hold the paper over a heat source - for example, next to a lamp;
I hope I helped you)