towel. A towel is great because it is a relatively small prop and doesn't require much storage, a steel tub requires much more storage but it is probably hands down my favorite prop of all time.
The steel tubs, can be found at home improvement stores or through actual prop stores. The perfect size is about an 11 gallon tub. Anything smaller doesn't give baby much room to move and anything bigger is just ginormous.
For bubbles, I use plastic Christmas ornaments broken in half. Just lay the bubbles around the tub and you have a bath scene. You can also use towels, rubber duckies, and bottles of baby bath wash.
This tub is one of my larger props that I have moved across the country a couple of times, but it is hands down my favorite. I just wish I'd had it when my little one was a baby.