There are several considerations for choosing the right filler for a hot/cold pack. Fortunately, the considerations for hot-pack materials are the same for cold-pack materials; the same substances that conduct heat well tend to perform well as cold packs. Other than heat characteristics, important features are microwavability, potential allergies or sensitivities and aroma.
Organic Base Fillers: Rice, Flax Seed, Barley and Buckwheat
Each of these is a seed, and each is well suited and often used for hot/cold packs. These seeds are all fairly fine and have enough weight to contour to body parts, so long as they're not packed too tightly. They can be microwaved, which is the fastest and most convenient way to heat them. Because they are dried, their shape will remain malleable after removal from the freezer; they won't become solid like a bag of frozen peas from the grocery store. They each have a distinct but mild odor when heated. The primary drawback to any of these materials is allergies.
Inorganic Base Fillers: Ceramics, Beads and Synthetics
If you think you may be allergic to all of these organic materials, you may consider ceramics or glass beads—even vinyl-based synthetic beads. Ceramics and glass beads are heavier than most organic fillers and they can become too hot. Synthetic beads, such as vinyl-based beads often used in stuffed animals, are unlikely to invoke allergies, but they may release small quantities of chemicals that cause reactions similar to allergies. If you suffer from allergies and are not sensitive to irritants, vinyl might be a better choice. Ceramic and glass beads tend to get hotter and they're more expensive, but they are an excellent filler.
Some people add one or more herbs to their fill mixture. Wether you find various herbs to have therapeutic qualities, you'll probably find some that smell good. Particularly with heat packs, adding an aromatherapy component can make recovery from inflammation and injury much more pleasant.
A small amount of essential oil can be added to the fill. Essential oil is a concentrated extract from a plant such as lavender. This is another way to add aromatherapy to your heat/cold cycles. To limit the aroma of the essential oils, or if you want to extract the oil directly from the plant, they can be emulsified into mineral oil.