One of the things I wish we could do during this cold season is put up the pool. Water is a huge sensory input for our children as it exercises a lot of the body. Seeing going out and playing in water is not an option we have resorted to the bathtub. Now my kids tend to be pretty messy being sensory seekers but I have found when I am unable to put up the sensory swing due to behavior issues and my hands are full that sometimes a tub of water will do in just a pinch. I don't normally add any soap but instead toys like what they'd play outside with in a pool. To avoid me getting wet I pull the shower curtain closed and allow them to play sometimes up to an hour. It seems when they get out of the tub they tend to be much calmer... at least for me.
There are also a lot of things on the market that you can do to enhance this wonderful time of water such as fizz balls (eventually I'll get a recipe for it), color changing tablets, and bubbles (nice cause you don't have a mess on the floor) just to name a few things that you can buy.
If the water is warm enough I have even read about getting a bucket and filling water in it and putting it in the freezer. When it's bathtime they get the huge ice cube out and put some hotwheel cars in the tub as a platform for them to play cars. Another alternative would be colored ice cubes to float in the water. Just make sure your kid doesn't get too cold in the bathtub if you play with ice.
One concern about being in the tub is slipping. If you don't have a mat in your tub I suggest you get one. Other alternatives would be getting some aqua socks that will grip to the bottom of the tub to help avoid slipping.