Storing riding lawnmower under deck
Since you say you have a ceiling system and no direct rain in this protected area, I would think you want a material that will pack down firmly so wheels of equipment and walking won't depress into the material too much. I have used 3/8" stone dust, which is a by-product of the stone crushing process mixed with 3/8" pea stone. This material is heavy, forms grade and packs well. You could also use a clean "packing gravel" which is a very coarse sand used under concrete slabs etc. Another good solution is to use a garage mat over the base to park heavy objects on. These mats are available at many box stores and auto supply places. Another good reason to use a parking mat is that oil or fuel spills can be cleaned up before they seep into the soil. You can also sweep it clean occasionally so it looks neat as well.  Good Luck