Activity requirements
You need a car (clearly) and a sense of direction helps too.

Cost
It costs as much as the price of the fuel where you're driving, which may not be too cheap, but it's still a load cheaper then flying at staying in hotels.

What Fiked recommends
I've been on a couple road trips and here's what i've learnt: Firstly, a GPS system is only any good if you've got the right maps downloaded to it, but failing that, a good map never fails. There's no need to plan a route or even know when you're going when you leave your house, but a guide book is a good idea if you want to make it up as you go along. Campsites are dotted around everywhere in europe, but failing that i've camped in the middle of a service station car park and woken up surrounded by people and still no one's really batted an eyelid. An MP3 player filled with music and podcasts are a great way to pass the time and having multiple drivers help when you're tired.

Best time to do this
The summer is all about road trips to me, but I still like to get away before the schools break up so that I can really make the most of the driving.