Updated for packs up to and include Eco Lifestyle!
Shelves are great decor items yet it can be difficult to decorate them because even if items look small enough, they may not fit onto the shelves automatically (by going into slots that are defined when the shelves are created). As of the release of Island Living, these items fit onto shelves. There are some shelves that allow for a few slightly larger items to be used. All of the shelves that have the ability to hold the larger items are also included in the build mode room. Find images of all of the items below.
Here are rooms saved to the gallery for different categories to make finding things much easier. The items that are sorted by specific packs were based on the pack icon on the item image.
Hidden objects like collections (including the Island Living collections items) sorted by base game and pack: https://www.ea.com/en-gb/games/the-sims/the-sims-4/pc/gallery/51627306A2A611E9849A31018BE268BC