If you are prone to motion sickness, please take your favorite medicine with you. The roads in the highlands are curvy and hilly and it is not fair to ask to sit in the front seat the whole time. Some days are worse than others (I think the day to Glen Affric is the worst) so you can plan accordingly. Please feel free to contact me if this concerns you.
Stugeron as given on cruise ships has been recommended. It is effective and non-drowsy.
Also, there is something available in Scotland that is not available here. It has worked for many people. You have to request it at the pharmacy. It is behind the counter. The label at the top left (white on blue) says Boots Pharmacy.