Fixed Objects are solid items that can not be interacted with. They mainly serve as decorations, and there are a wide variety of different types of them. However, you can disguise a gameplay object as a fixed object if interaction is necessary. Also, there are handheld objects, some of which, ironically, can not be held. For the most part, though, you can hold them.
Almost all fixed objects make an appearance in vanilla spore with items like fossils, geodes, scrolls, books, lamas, tribal huts, colony buildings, etc, etc. However, Roughly half of the plants in the fixed object section are not found in vanilla spore and are specific to galactic adventures. Items such as the beach ball, barrel, and the cannonball are examples of objects that do not appear in any stage of spore.