Some sites with very good info on inexpensive bathroom renovations:
Paint does wonders! New shower curtain, fluffy towels and rugs. If you want to take it further, update the lighting, flooring, hardware and accessories.
Paint !!! New floor , I used a peel and stick textured wood look vinyl planks, it only cost me about $ 30 !!!! Are a new shower curtain , New grammar , even new pictures!
The cheapest redo for any bathroom is a thorough cleaning. Followed by a dose of grout sealer. Updated light fixtures.
I did a bathroom makeover and you can read all about it here >> https://www.homeroad.net/2018/04/a-neutral-bathroom-makeover.html
I fixed up my vanity, got a new shower curtain, made a new mat, and painted the walls so the whole bathroom looked new!