Everything here is fairly straightforward. As previously cautioned - don't put things meant to be crispy in the microwave, in this case that category includes the flatbreads and the spinach-feta pastries. Reheat these incovered on baking sheets in an oven or toaster oven at 350-degrees.
If you've got an oven going and want to heat the moussaka and pastitsio that way, they will also taste good, but cover them with foil in a baking dish so they don't dry out. Alternatively, they can be reheated in a microwave, second (or third) best would be a covered skillet.
The rice and the potato salad should be heated until just warm, microwave is easiest.
I recommend keeping both cakes in the fridge. I prefer the walnut cake cold, but both could be heated by a short spell in the microwave. If heating the walnut cake, I recommend adding a scoop of vanilla or cinnamon ice cream on top once it it warm.