There are a few factors that come into play. I will try and provide what they have and how to get around them.
Locked airport...
Most airports I have been to have some form of a fence, gate etc so you would need to get past that first. Chances are there are also cameras but a ski mask and gloves will help you out with that. Smaller fields may have less protection and you may even be able to walk right up to the hangar. But you may draw more attention to your self as the people at the smaller fields may know each other better and know that its not your plane.
Locking the plane up...
Some GA planes are hangared in presumably locked hangars. So the first thing you would need to do is get into the hangar. In this case we are talking about a door lock or a pad lock of sorts which can be generally opened with this if you practice a bit.
Ok now you are in the hangar/have the hangar door open...
Most General aviation planes do have door locks form what I have seen. These locks tend to be of the smaller key type (think pad lock key size) but Im sure they vary and newer planes carry larger keys. Most of the locks I have seen can be picked with something like this. There are types of locks that are better at preventing picks (like tubular) but they too can be picked.
Ok now you are in the plane...
Maybe the plane has a prop lock (you can get that off with a bolt cutter, but don't nick the prop).
Maybe the plane has a throttle lock, back to our lock picker to get that off.
Now you need to start the plane up. Here you have a few options one of which is to use the lock picker to turn the ignition key. Personally I would not do this for simple flight safety reasons. In this case you would bring along 3 switches with wires hanging off of them (this would work better on older planes) get under the panel and splice in some bypass switches. This would allow you to control each mag and the starter. You could also try a simple flat head screw driver and some good old force to break the tumbler and operate the starter. On newer planes with sleeker one piece dashes you may have an issue getting being it to hot wire it but I would think there is a way.
Ok now the engine is started....
The plane is running and you are taxing out, assuming no one in a nearby hangar asked you what you were doing and where Bob (our fictitious victim) was and why you were flying Bobs planes. No one is going to stop you on the taxi way or from the tower as far as I know. Now you are in the air, flying VFR to somewhere. Lets say Bob still has no idea his plane is missing since he does not intend to use it all week. You get to your home base airport with the plane and still no one is wise to what you have done. Maybe its even a private strip and no one else is there to say anything.
Now you have the plane in a "safe" place....
The first thing you do is get the plane out of sight, put it in a barn maybe, get out your aircraft paint stripper and get rid of that N-Number would be your first task. Maybe strip more of the paint so as to make the plane harder to identify. Now you are thinking you will do one of two things with the plane, use it your self or sell it.
But you need an N-Number...
The first thing you will need to do is get a new N-Number for the plane this is where you will hit a problem as the airframe is already registered to Bob so you can't use that airframe number. I am under the impression that this where the road will end for you as its harder to fake an airplane number since they are far more closely tracked than say a car vin where you can use one out of a wrecked car (to an extent, its still illegal but technically can be done) There may be salvation laws for planes as well that you can exploit here if you can get a wrecked version of the same plane.
But Can I Sell It...
Maybe you can part it out and sell it, plane parts do command a high price. But with out the log books you may have an issue proving the time on a lot of the parts. Chances are Bob did not leave his aircraft logs in the plane but maybe he did (bad idea Bob). Now you can just part it out but you cant sell the whole plane. In trying to do that someone is going to be asking about the lack of N-Number and the log books. But hey, you tried.
What about a joy ride...
Well sure, we have already established that you can get into the plane and fly it away, you could very well return it (with a bunch of broken locks) to where you found it.
The only question is can you fly the plane...
Remember every plane is different, while you may be able to saunter into the cockpit of some pretty heavy metal you may have no idea what you are doing or how to get it off the ground. I have no idea how to start a jet engine so I doubt I could just hop in one and fly away no matter how accessible the plane is. Lets say you have used X-Plane a bit to much and know how to start up and (theoretically) fly the plane, everyone here knows that theory and practice are not the same thing. Even if I got into the cockpit of a running jet I would still need to actually fly it. So on some level you are limited by experience unless you are a well trained pilot, well trained thief, and in general a criminal.
Please note I do not condone or endorse (or have ever been involved in) aircraft theft in any way. For the sake of argument I am presenting the facts to answer a hypothetical question. If for any thing, don't ruin Bob's week....