Something is odd when it’s out of place and not easily dismissed. Developing a theme of some sort which infers some seemingly random items are somehow connected would add a sense of intention.

If your friend is in to fishing then you could litter fishing related things about their home. If they’re into a certain spots team then leaving a rival team’s merch would certainly catch their attention. You get the idea.

If you want to concoct something more elaborate and strange pick an object to start with and think of the most unrelated thing you could still associate with it. Like an eagle feather leading to Xavier: Renegade Angel. Going on a wiki-walk is great for this sort of brainstorming. Identify a bunch of loosely related but inexpensive items that are easy to hide and simple to find.

The “doubles of stuff they already have” bit is gold though.