My second favorite thing to dehydrate, so far, has been apple slices. If I slice them thin enough, and dry them long enough, they come out nice and crisp. When stored in an airtight container, they seem to get more crisp over time too.
1 lemon (or 2 - 3 lemon cubes)
About a dozen apples (depends on dehydrator size and apple size)
1 tbsp sugar (try lemon sugar or vanilla sugar)
1 tbsp cinnamon
dash of nutmeg or cloves (optional)
dash or two of salt
Cut the lemon into quarters or melt the lemon cubes in a small dish.
Scrub the apples, then cut 3 of them in half. Place them cut side down on a cutting board and slice them horizontally, across the core. Use your best knife and slice them as thinly as possible, 1/8" thick or less. The widest side of a box grater works too. The apples should not need to be cored as the seeds will fall out with such thin slices. Just remove the ends, seeds, and any larger core pieces with the tip of the knife. Don't worry too much about the core fragments, most of the smaller ones will dry right up and not be noticeable in the finished chips.
However you get your slices, place them in a large non reactive bowl (glass or plastic) as you go. To keep them from browning, after you cut the first 3 add 1/4 of the lemon juice and toss to coat. Add another 1/4 with the second 3, and so on until all the apple slices have been coated. Finally, add the sugar, spices and salt at the end and make sure the slices get evenly coated with that too.
Arrange the slices on the dehydrator trays as close together as possible, without overlapping, since they will shrink. Dry them at around 135 degrees for up to 2 days until they are very dry and have a bit of snap to them when bent. It's ok if they bend a little, they will crisp up over time.
These are addictive to eat like chips. They are also good to crumble up and mix with dry oatmeal to make homemade instant flavored oatmeal packets. They keep for at least a couple months in dry airtight containers.