We all agree that brownies are awesome, right? Right. (Well, many of us, anyway, though I have yet to meet a brownie-hater.) But now that there are so many unusual chocolate products on the on the market, from filled and infused bars to flavored hot chocolate to exotic bonbons, it's understandable that you'd get the urge to mix in something aside from walnuts from time to time.
Add Some Salt
My go-to ingredient to make anything more interesting is salt. It's fine by me if you just want to toss some fleur de sel or Maldon salt into your brownies; I find the extra crunch and salty-sweet combo delightful. But you don't have to stop there; pretzels are a natural to crush up and mix into the batter. You'll want to use big, crunchy pretzels—Martin's are a favorite, as are Snyder's—and they don't keep quite as long since the pretzel tends to get softer over time. You should be fine for about a week if they're wrapped airtight. They'll totally be around for a week, right?
Bacon is another way to get some salty-crunchy goodness, and in many mainstream sweets nowadays; the real trick is to get the bacon super-crispy before you add it to the batter (cooled, drained, and crumbled, of course). I've never done it, but you could candy the bacon ahead of time too; I'm almost always in favor of candying things.
Make Them Nutty
Of course, any kind of nut imaginable is appropriate for a brownie, but why not make 'em spicy? Chile and chocolate are a natural fit, and any leftover spiced nuts are great to snack on. You could just toss in a little cayenne if you're looking for a kick, or chipotle if you want something mild and smoky.
Peanut butter brownies will make almost every kid happy; Peanut Butter & Co. even makes a bunch of flavored peanut butters for when you're feeling adventurous. If there's an allergy issue, why not mess around with other butters? Many larger and specialty grocery stores carry different kinds of nut butters, like almond, cashew, and even sunflower seed. You can also buy them all online, natch.
Chocolate-on-chocolate is never a mistake in my book. If you want to go ahead and toss in a couple handfuls of chocolate chips—milk, dark, or white—I say go for it. Nibs are great in brownies, too; they're a really nice way to get more chocolatey flavor in without the extra sweetness of chocolate chips.
If you want to get out the big guns, though, I'd recommend getting your hands on some double-dutched or black cocoa powder. If you decide on the latter, you'll want to use it as a complement to your usual cocoa powder; an all-black-cocoa brownie won't taste very interesting. But the addition of it, or double-dutched cocoa, give a richness and depth of flavor you really can't get from anything else.
Of course there are scores of other options, many more than I have room to write about here. I'm definitely interested in finding out what SE readers add to their brownie recipes —bring 'em on!