Skullcaps can be worn under the helmet. These tend to be quite snug and warm as well as water resistant, really good for winter. The vents on a helmet will still cool the head.
An over helmet cover can also be good, does tend to get hot in my experience, tends to be the favoured option for those that have long hair. Water can run off and miss going down your neck, which has happened when I've worn skullcaps.
Have a look at a traditional cycling cap too. These can sometimes be resistant, plus the low peak can help keep the water off the goggles.
Or take a towel, LOL!