Theres nothing that can be done thats worth doing. If the dents were severe, then a good framebuilder can easily replace a tube. However then you've got to repaint the bike.

Trying to remove small non-structural dents in a bike tubing ain't worth it, it'll cause distress to the tubing and probably mess up the paint around the repaired area.

Would it be possible to fill the frame with water and freeze it with the expanding water popping out the dent?

…yeah, thats possible, because the water knows just where to push out…

Barbara, or her sister?

shes has more than one sister, but yes Louise Mandrell, that useless piece of garbage, shove her in and hope she never gets out.

According to physics, pressure expands in the direction of least force. The pressure created by the expanding water force will be uniform around the entire pipe. Since he dent is the weakest part of the pipe (why you want to get rid of it), it will deform sooner than the rest of the pipe and push towards its original position to release the pressure.