Either sand or pea gravel is going to have a greater density than rice, which will change the character of the exercises. I doubt you'd be able to push your hand into either as easily as you can with rice. The resistance you'll have when you try to spread your fingers or move your hand side to side from the wrist will be greater. I use oiled sand to store my garden hand tools in (keeps 'em from rusting) and no way could I do rice exercises in that bucket.
Sew the rice into long and narrow cotton bags and send it to your girlfriends in the cold north, as a microwaveable bed warmer. Women love that kind of stuff. Then you can think about all the energy they're using instead of wasted food.
is aquarium gravel made out of rock of some sort? if so, it will probably have basically the same density as pea gravel (maybe more actually). you might be able to find some sort of plastic beads that are used for filling little bead pillows at an art store or something.
Density???.... what's really important here are the surface properties (friction, particle shape, angularity). Even if pea gravel or aquarium gravel has the same density, no way it's gonna have the same surface properties and no way will you be able to push your hand into it as if it were rice (which is rounded and often polished). Plastic beads better (surface properties more like rice).
Don't feel bad about "wasting" a couple pounds of rice. Very little nutritional value anyway. And if you compost it you're not "wasting" it.
WTF? You're going to boil (aka sterlilize) the stuff to eat it anyway, just use it for your exercises, then cook it up with curry afterwards. Wash your hands before the exercise if you're super concerned about it.