I am on a long trip at the moment. But long or short, I only bring enough clothing to last a few days. If I could wear everything, I could go about 4 or 5 days depending on how many undergarments I bring along. That is usually the limiting factor. Usually it’s 4 besides the ones I wear leaving home. TMI? Well get over it. One way or another I am going to do wash on my trips. Usually before the day I would otherwise run out of clean clothes. But how?
First of all, one advantage of hostels is that this is not an unusual need in those places. Most hostel guest do laundry often due to their not having a lot of clothes packed away. So many will provide a service of doing your laundry for a reasonable fee. Or at least have machines and soap available. Ask. And sometimes this info is available online too.
But if you dont have this option, then be prepared. Which means at the least having some small amount of soap and maybe a bit of parachute cord for hanging up things to dry. Do it in the sink, or shower while showering.
Let me add one possible item, which you might want to have along anyway. (Most travel gear should have more than just one function). Consider a medium sized lightweight dry bag.
It should be sealable. And if designed so that water cannot get in, then water can’t get out either. You can just use the sink wherever you are staying. But you don’t know where that sink has been. This offers up a level of private sanitation which even the laundromat can’t match.
Fill it up with your clothes, add soap (I bring Dr Bronners pure Castile liquid soap, peppermint usually) and water, seal and slosh around, empty and repeat for rinse. Hang and dry. (Turn bag insides out and let that dry too.) If the hostel provides soap then use that. They often do.
If you’ve invested in synthetic fabric clothes, by morning they will be dry or dry enough to put back on damp. Damp clothes will dry faster wearing them than just hanging.
And what else do you use a dry bag for? To put away sensitive things like cameras and electronics when rain threatens (or for some reason you are fording a river). Or at least you can use it to store away the stinking dirty clothes before they are washed.
Single function items, however cute or gimmicky, should be left at home.