I have a minivan I drive around to appointments nearby but do also have a 4 door sedan that I use frequently. When I take my sedan I always use a rinseless wash and just take a 5 gal bucket of water with me. That way I can refill my sprayer I use to apply the rinseless wash a few times and still have water on hand to refill any spray bottles, dampen carpets for cleaning, and anything else I need water for. Usually a 5 gal bucket will carry me through about 3 or 4 details depending on how bad the carpets are. I do also take a smaller bucket with me so that way I can pour water into the smaller bucket to dip rags into for carpet cleaning.
Whether I take my van or my sedan I usually just take about 2 plastic bins of supplies with me now which stores 3 or 4 polish options, and a few different waxes, about a dozen or so towels, extension cord, polishing machines, metro vacuum, and a steam cleaner. The bucket of water can ride shotgun with you and the rest can easily fit into your trunk and back seat.