When we go on family rides, I have a section of high tensile chain and a keyed disc lock. Find an object that is fixed solid to the ground, chain them all together, and the chances of anyone taking your stuff is pretty slim.
Also, if using a bike rack to secure your bike, put the back tire in, not the front. If you chain the front tire in, the thief will simply unbolt the front tire, find a nearby bike with an unsecure front tire, and voila, they have a bike. By placing the back tire in the rack, you have access to the seatstays that can be cabled to the bike rack. Not impossible to steal, but alot harder. We had this problem in Turkey with highend American bikes getting stolen and this was the preferred mothod of theft. My buddy came out to find nothing but his front tire chained to the rack. Right next to his bike was one with no front tire...........
But like others have said, a determined thief will get what they want. The only thing that is absolute is that nothing is absolute.