We call this a Monster Hunt. It's a Halloween themed riddle scavenger hunt. We write little rhyming clues that we hide around the house and yard, and the last clue leads to some kind of themed gift. This year it was the coveted Welcome To Night Vale book. When my kids were younger it might be a Halloween picture book, cool costumes, or some kind of dark and spooky decoration or toy.
These are our family's clues, things my kids' dad and I knew they would be able to figure out. They refer to things like our favorite Halloween books, decorations we have around the house or yard, the dollhouse which my kids decorate each year, the refrigerator, the costume bin, and one year to a volunteer pumpkin that we had growing in our garden. You might be able to use some of these clues as is, or change them around, or just read them and get your own ideas altogether.
Each year, inside one drawer of their Halloween Advent Calendar, is a little piece of paper that tells them today is the Monster Hunt. (Follow the above link to see what other ideas we use in our advent calendar.) I hide all the clues, being careful that the order they're in makes sense, one leading to the next, and that the last clue leads to a spot that's big enough to hide whatever their gift is. The kids are so excited, and always love this little Halloween tradition.
This was the first year that my youngest was able to read the questions himself. Each time they found a clue, he'd settle in and make himself comfortable, then read the clue aloud to us, before running off to the next location.