Adventure has a power tonneau cover that rolls away and stores itself. Explore has a "manual" cover that you must physically lift yourself and it's 4 panels that you have to deal with putting somewhere if you want the bed open.
Adventure has "gear guard" which is two locking cables in the bed that let you lock around stuff like bikes or whatever. In addition to a little physical security, the cables and a camera that watches the bed will detect someone trying to mess with your stuff and Rivian will alert your phone. Explore does not have gear guard.
The Adventure has a much nicer audio system. The bluetooth speaker is included with Adventure -- unknown if Explore will have it, or not.
The interior materials are nicer in Adventure, compared to Explore. Real wood dash, and microfiber headliner, for example. Compass Yellow accents, as well. The floor mats are different, as you pointed out.
The Adventure has passenger lumbar adjustment. Explore does not.