A friend of mine rented one of the conference rooms at the hotel in Mason next to Graeters (Hilton??). I don't know what the cost was but it was a nice sized plain room with plenty of tables and chairs, access to bathrooms, etc. Seems like it would work well for a shower.
We had a friend of mine at Claddagh. It was in a private room and was super nice. I am not sure if there was a fee for the room or not it was about 5 years ago so I dont remember. Also depending on what time of year there are tons of parks I know you can use. In Mason I think you can rent out rooms at the mason community center as well. Good luck!!
Re: Shower site recs?