Another vote for p-clips. Or a saddle bag. Or front panniers.
I'd avoid a seatpost rack - if only because a saddle bag seems like a more sensible alternative to having the load cantilevered out over your back wheel.
If you have a decent hydration-type backpack then you can use that for at least some of the load (I'm assuming that you are basically looking to carry a few clothes, a wash kit, plus tools and spares). This will probably prompt someone to issue some dire warning about how if you ride with a backpack really really really bad things will happen to you, but mountainbikers ride all day with backpacks - providing you don't overload it and provided you don't have back problems you'll be fine. But yes more complicated and expensive solutions are available.