Anything aromatic would work well if it can stand being heated. I use rice mainly in a 6" strip folded in half and sewn into a tube, then turned inside out and made into tunnels to distribute the rice evenly, and found it works great.
My only problem has been keeping my children out of them so I can use them! I just whipped up 8 of them last weekend to get a supply of them on hand for our aches and pains this winter.
2 minutes is too long in my microwave, but the heat will last for 20 minutes direct heat and 40+ minutes indirect heat. For the first 10 minutes out of the microwave, I usually have to put the bag on another towel to cool it down so I don't get burned.
Nice thing is that the bag works in the refrigerator just as well when things get hot, so it is ideal for muscle aches.