First things first - a big welcome. Exciting, first trip in a RHD car in mainland Europe!
Some tips from the rac...
http://www.rac.co.uk/drive/travel/driving-abroad/when-you-get-there/ All good sound advice.
I have driven in both countries many times in a RHD car. Generally speaking road manners are good and the std of driving is better in both countries than the UK especially where lane discipline is concerned. They do tend to drive faster than we do and in rush hour traffic dart for any available "space". You will have to watch for that in the cities - as a first off experience I would try and avoid big cities at these times - Amsterdam, Paris (I know that's a bit south from where you are planning) but just in case and give you a scale perspective. A watch word? Drive defensively.
Bikes are much more common in both countries compared to the UK. In France, should there be contact with a cyclist the law considers it your fault no matter what the circumstances. I am not sure if this is the same in Holland - but there are many more bikes in Holland. Many! Be aware.
In the countryside you should have no issues except older French drivers who still think they have priority from the right (an old law).
There are two big things to get used to IMO. 1: Round-a-bouts. You go round them in a anti-clockwise direction as opposed to a clockwise direction here. Once on the round-a-bout you have priority. So give way to the left on entering one. Don't worry you will get used to it quite quickly. 2: Overtaking. You are on the "wrong" side of the car and cannot see past the vehicle in front. Be prepared to take your time and don't be hassled by other drivers behind you. Go steady and try to avoid being in a hurry.
Remind your front seat passenger that when they get out of your Cali they are potentially getting out into a traffic lane should you stop at the kerb.
Don't forget all the warning triangles, first aid kits, breathalizer kits, hi viz jackets, GB sticker/external vehicle country ID etc. You can find country requirements on the rac web site. Oh, and your dipped beam headlight adjustments. Sat Nav is a must - far less stressful for all!
Hope this helps. Enjoy your Cali experience.