We got a car in our hotel (Los Fariones). Opel Astra €30 per day - I think the rates were better than online booking in advance. This time of the year is quiet enough.
Car hire - wait 'til you get there. Boards - I don't know about. If you're staying in P del Carmen you probably can hire them somewhere out in Matagorda where surfers etc hang out.
Used carjet.com on 2 occasions in Lanzarote previously. Was around 160euro for a Peugeot 206 size for a week in Aug/Sept. Full insurance covered - collected at airport on arrival.
hired a car in lanzarote about 10 years ago. behind the lanzarote sol apartments in matagorda. suffice to say the car had plenty of bumps etc when I got it and when I went back into the office to let them know they just said don't worry and when I left it back and was explaining to the car hire person (there would be a different person on duty) she just laughed and said no worries. in portugal the following year they nearly took a magnifying glass to the car. enjoy the driving, its a real pleasure on Lanzarote.
Two weeks ago we rented a Ford Focus at the airport in Lanzarote for a week for €160 from Hertz. Other companies at the airport offered small cars for €100 a week.
There were plenty of boards available too from places such as