Construct or upcycle a rustic shelving ladder as a novel but useful feature. It can be used to hang towels and store bath products, and would cost little to nothing.
You can go one step further and build shelving from reclaimed wood to match your new storage ladder for a farmhouse style.
Use old jars as holders for things like toothbrushes, plants, or anything else that needs a home in your bathroom. You could even paint them to match the color scheme of the room.
Use rope to wrap things like the waste basket or exposed pipes.
Paint old cabinets to establish an overall style for the bathroom. If you are adding rustic wood touches, then stick with a farmhouse palette such as a subtle blue.
Buy faux stone veneer to use as a bath surround and/ or backsplash. It is extremely easy to cut and fit and is easily removed if you fancy a change later down the line.
Collect pebbles and stones to create a unique shower floor as a low-cost alternative to tiles.
A cleaned up wooden pallet is a very versatile item that can be used for a range of DIY projects. Perhaps add clothes hooks to one, with a nice, clean, white frame to hang on the wall as a towel rack.