Sometimes it does get boring for him so I switch up the things he does in quiet time. If you have any ideas to add, please leave them in the comments!
1. Puzzles. Depending on age, this can be wooden or cardboard puzzles
2. Train track and trains. I give him the wooden train set with trains and it's something he can't normally do with his little brother knocking it apart.
3. Coloring books and crayons. Simple and fun.
4. Tag reader and books. It is a little expensive to get the stuff, but eBay has great deals!
5. Put away their clean laundry. My son just turned 5 and he loves to put his laundry away.
6. Create a collage from magazine clippings. If trustworthy with childproof scissors and a glue stick allow them to do it all in their room. If not, help them cut out items and give them tape instead.
7. Organize their room. I let him choose where his things go in his room and he organizes them how he thinks they should be organized.
8. Miscellaneous bin. I have a bin of miscellaneous items and they are only played with every once in awhile in quiet time.
9. School books. My 5 year old loves to preschool and kindergarten school books where he can trace letters and try things on his own. Then he shows me what he did afterwards.
10. Learning tasks. If he is working on something such as writing his 3s and some letters the right way I will print off some special sheets for him to practice. He likes it because it gives him time to try it on his own without anybody looking over his shoulder.