A lot of what you're asking has to do with where you live. If you live in a small town with little crime, then maybe a cable lock would do. For anywhere with significant bike theft, it will be unacceptable, sadly. I had a bike stolen that I had locked with a cable- they cut right through it. I wised up and now use a U-lock. Unfortunately there's no perfect solution. The U-locks are bulkier, but you kind of need them.
As far as the seat and wheels go, you can take off the front wheel, then lock it with the frame and the rear wheel all together. That would be the most secure solution. Alternatively, you can replace the quick releases with regular bolts. (which might stop a crime of opportunity, but a thief would just need a wrench anyway, so..) As far as the seat goes, there are little cable locks designed to secure your seat to the frame. You could also replace the q/r with bolts for your seat. Again, the necessity of this depends on where you live and how nice your bike is.
Don't skimp on the lock.