I'll assume half a dozen people have said the spinning wheel? My plan would be to get a job as a shepherd and use the copious amount of time to work out how to construct a spinning wheel. (maybe even a portable version at first).
Regarding sewage systems. People tend to underestimate the importance of flushable toilet paper.
Without it you have two options: Dry storage and removal of wiping implement (modern system), which partially defeats the purpose of plumbing since you still have to the smell and going around and manually collecting/ disposing of waste.
Reusable wiping implements, (which the Romans had) which couldn't always be personal or perfectly sterilized (they used vinegar, which is a disinfectant, but not one I would trust to stop cholera).
Finally the waste was valuable (before the invention of modern fertilizers), which meant that you could cheaply (or freely) get people to collect it. The ideal sewage system doesn't flush waster into the ocean (even if there isn't enough to contaminate the nearby waters), it captures it in a way that can be used for fertilizer (and I think it was undesirable for it to be composed first).