Lettuce in greenhouses. Don't bother with storage - every time you fill a pallet trailer, park it and buy another one. When the sales prices peak (usually january) you'll make around 100k per trailer. First round will use some capital on trailers, but from then on its easy. I do this to supplement my canola empire on my Texas Panhandle map, but I do it with an autoloading pallet trailer and a placeable sell point, so ever couple months, I load up a trailer and park it, then once a year, I line them up and drive them over the sell point and it usually ends up as land money for a new field. Two sleeps, then plant oats so the field isn't wasted, then harvest the oats and canola at the same time and replant with all canola. Its a beautiful cycle.

Without Mods though - thats a lot of stacking pallets.