nat,
"Zip" towel. Since you can change it easily. Just wear cotton T shirt is OK. If wear背心, difficult to change because you will not bring a lot of 背心 when you going out.
Yiu Yiu has a lot of 連身 body, and she is now wearing as 底衫, it is very good no matter for sleeping or daytime, 唔會凍親個肚仔 no matter how large action they are doing.