Add-ins. I like to add coconut and cocoa nibs to my granola. Also, use a different citrus zest, spices, combination of nuts and/or dried fruit. I do highly recommend using sliced almonds (no matter what other nut you add) because they provide a crunch that is amazing and can't be achieved with other types of nuts. 

Drying the oats correctly. This is the key to a crunchy granola. The oats are impregnated with a mix of olive oil and maple syrup, that is warmed with some brown sugar. This sticky mess is then baked at a low temperature (very important) to dry the wet mixture, resulting in a caramelized granola, every part coated until it hardens.

Slow baking. It's important to give it the time it needs in the oven. If it doesn't dry it will not keep well, and it will be chewy without being crunchy, like a piece of regular toast vs an oven dried toast, the latter keeps for weeks.

Baking pan. The type of pan/amount of oat mixture ratio is important. I use sheet pans or cookie sheets, so the granola is spread as much as it can and is more easily dried before it colors too much. A golden color is great and good for the caramel flavor, but it should be dried further. If you put too much oat mixture in a smaller pan it will take too long to dry out in order to stay crunchy.

When to take it out of the oven. Follow the time given in the recipe, and stir it a few times during the baking time. It will still feel wet when you take it out of the oven, but it will harden as it cools. You'll get the hang of it when you start making it regularly. Because you will. One batch of homemade crack granola and there's no turning back. You're spoiled for life.

Oats. I recommend the traditional or rolled kind. They're sturdy and have a great crunch. Instant oats just don't work as well.

Gluten free. Use certified GF oats for a gluten-free option. You can buy it online: Quaker Gluten Free Old Fashioned rolled oats or Anthony's Organic rolled oats.