The simple answer is, almost anything! The only exception is fat, which tends to spoil, so avocadoes are not the best produce to dry.
Suggestions for fruits and vegetables that dry well can be found on the shelves of your grocery store. Grapes (raisins), plums (prunes), apples, pears, peaches, apricots, pineapples, bananas, tomatoes, and herbs of all kinds are popular options that dry very well. So are cherries, dates, nectarines, papayas, strawberries, and onions. Find out what sorts of dried foods you like best by experimentation! It is always interesting to take a new food and find out what it looks and tastes like with most of its water content removed!
Dried produce tastes great alone, but can be eaten many other ways. It can be re-hydrated by simply adding water. It can be ground into powders and used in soups and sauces, or flavorings in other dishes. Fruit can be processed in a blender, spread on a sheet of plastic, and enjoyed as fruit leather. You can experiment by mixing different fruits for different flavors of leather. Though dehydrated fruits taste great alone, for a change they can be candied in a dehydrator with honey or sugar for a special treat. You can make your own onion powder spice by dehydrating and pulverizing onions. In fact, all your excess garden herbs can be dried and added to your spice rack.
But dehydrating is by no means limited to fruits and vegetables! You can also dry meat and fish to make jerky (with certain extra precautions, of course, to prevent their fats from becoming rancid). In fact, many people buy dehydrators because they want a way to preserve meat during hunting season. Grains, nuts, and seeds can be dried to increase their shelf life. Fruit-flavored yogurt can be dried to make a kind of taffy. Dehydrators can be used in the making of yeast breads, cheese, and yogurt. You can make granola or dry bread crumbs. It is also possible to re-crisp graham crackers, chips, and other bought items should they become stale.