T.G.
My 23 month old daughter went through the same thing a few months ago. While it is hard to enforce, it is a safety issue having her stand in the tub. We now tell her that she must first sit, then she can have her tub toys. And we change them up every once in awhile so that she gets "new" ones to play with. The crayola bath crayons are great, they wash right off and she must sit to use them.
While I agree with the other moms that she can not stand in the bath tub, I do not agree with the tone of some of the postings. This can be a difficult situation, especially because you don't want to do anything to make her afraid of the water. It is a safety issue, but every parent goes about how to get it done differently. Good luck!