For a rainy-day version, choose a short route through two or three shared spaces or a few clear objects in one room. Use scrap paper, one clue at each stop, and a small final surprise or a family activity as the treasure. Put the first clue out when the play area is ready.
A quick example is a start card at the rug, then the bookcase, toy basket, and a final note that points to a shared treasure spot. The route can be done entirely on one level and does not need extra decorations.
Treasure Hunt Box supports creating a custom route and map, generating or customizing clues, and printing the hunt to hide and play. You can also make the same small route by hand.