Heat the oven to 350F degrees. Line a large rimmed baking sheet with parchment paper or a silicone mat.
Spread the oats and nuts (if you are using) evenly on the baking sheet and bake for 8 - 10 minutes, until lightly toasted. Transfer to a large mixing bowl.
While the oats and nuts are toasting, combine the butter, brown sugar, honey, water (which helps the sugar to dissolve), and salt in a medium saucepan. Cook on medium heat until the butter is melted and the sugar dissolves. Remove from the heat and stir in the vanilla.
Pour the mixture over the oats and nuts and stir with a wooden spoon or spatula until all of the oats are covered. Then mix in any of the other mix-ins, except for chocolate chips. If you are using chocolate chips, let the mixture cool for 10 - 15 minutes before adding, so the chocolate doesn't completely melt. You may have a little melt, but that is ok as it acts as an extra binder for the oats!
Line an 8-inch square baking pan with parchment paper or wax paper (my preference is wax paper, but if you use parchment paper, spray lightly with cooking spray). Use enough paper so that it reaches the top of the pan. This makes it easy to lift the granola mixture out of the pan to slice.
Pour the oat mixture into the prepared baking pan. Using a spatula or the bottom of a lightly greased measuring cup, firmly press the mixture into the pan evenly. Do not skimp on this process! This makes sure the mixture can be cut into bars that stay together and don't fall apart!
Chill the granola bars for at least 2 hours. Lift the bars from the pan using the edges of the paper and place them on a cutting board to cut. Use a very sharp chef's knife to cut the bars. You can cut evenly into 12, 4-inch long bars, or any other shape or size you prefer. Serve and enjoy!