Depends on where you go in Spain. Go to a less touristic place and there's nothing to be seen, go to more touristic place and well, a lot of people and then some more. Also depending on how far south you go they tend to be a bit stricter in the northern part compared to the southern part.
Renting an offroad bike is easy, finding place to go is harder.
I did it like this, went to Escala (I have an appartment there) did a guided quad tour, rented an offroad bike, and there you go....you're set to do some offroad adventures in places you know it's allowed and then you can also just go exploring and when you get caught, just learn the words -> Me he perdido (me he perdiédo) (I am lost)
Places I recommend are
L'escala
Pals
Torreblanca
Alicante
Places to avoid
Benidorm
Barcelona (seriously keep away from that city ESPECIALLY on a motorcycle, when you're at a red light they don't go when it turns green, they go when it turns red for the other side)
Annyway, have fun there, just don't get caught by the Guardia Civil (Benemérita) Spanish police force with a military function, not like the local police, Guardia Civil are nasty fucking bastards.
Oh I've been going to Spain since I was a kid, have been going there for 30 odd years so when I say to watch out for the Guardia Civil I mean it, have had contact with them on more then one occasion.