Danny:
Not sure if I would call it "joinery", but you might consider putting a threaded insert into the leg at the point where the shelf would meet the leg, and on the bottom of the shelf rout out a recess to accommodate a countertop "tite-joint" fastener, essentially a 1/4-20 bolt, with a 1/2" x 3/4" metal bar that has a hole in it. It will pull up very tight. You could put the shelf in after the assembly of the table, and it will be at least as strong as any joinery that does not cut a dado, or something like it, into the leg.
If you are not going to stand on that shelf, it should be sufficient to hold whatever one might put on the shelf. Sometimes woodworking is less about joinery and more about proper fastening. Unless, of course, one is a woodworking purist, at which point you might consider a sliding dovetail. Nightmare assembly, as noted above.
Perhaps when cutting the legs, you might create a ledge where the shelf will be, making the leg slightly thicker/deeper under the shelf. This would help curtail the larger failure point of any joinery, which would be downward pressure on the shelf, and not weaken the leg by cutting a notch in it.
Good luck with whatever you decide.
Tone