Word to the wise, be careful with those Atkins bars. I recommend eating half of one as a start and see how you tolerate it. The sugar alcohols in them can cause some explosive digestive upsets, so it would be a bad idea to eat one and head out on a car trip until you know how they affect you. They do taste pretty good though. I bought a box once and liked one so well that I ate a second one. That was a rough morning at work, lol.
ETA: The Think Thin bars are good, might be worth a try.
If it doesn’t specifically have to be a bar, I can recommend Lenny & Larry’s cookies. I’ve tried all of them except the coconut one (because I don’t care for coconut) and they were all very, very tasty.
ETA: Oh, and the brownie was excellent as well.
This, for anything made with Maltitol or another sugar substitute. They can cause you to make impolite sounds and smells in company.
Thanks for the heads up, as it were. 
I’m always a little wary of sugar alcohols.
Thanks for the suggestion, but the fiber is really low (1 gram) and there’s still sugar alcohol in it. I think I’ll pass on this one for the moment.
Winner! This is genius.
This is near perfect. Thanks for thinking outside the box even if I’m the one who was thinking too narrowly when I created the OP. I didn’t even think of a cookie for breakfast. I’ll still be looking to try many of the bars in this thread because options and variety are always good, but I’m liking this cookie idea.
A cookie for breakfast. How great is that?!
I like that it’s vegan and that the protein comes from peas and rice.
My only small issue is that it was hard to find based on online sources. But I got around that. There’s a company called Munk Pack that has pretty much the same cookies as Lenny & Larry’s, but they’re easier to find. Munk Pack also adds green tea extract and monk fruit, which is nice. The protein is also higher in the Munk Pack cookies. It’s a tiny bit on the pricier side but half a cookie has the same protein and fiber as most bars.
Thanks again for the suggestion.
Almost never my goal. 
Lenny & Larry’s cookies are good (partial to the chocolate chip and the double chocolate, the snickerdoodle a close third), but they have more sugar than I like. The grocery stores around here usually have them, but at the local Academy Sports, they’re usually around 20 cents cheaper, surprisingly.
Quaker used to make these little oatmeal bars that I liked for their totability and yumminess, but I haven’t seen them around in a long time. Great thing was, you could pull open one side of the package, pop it in the microwave for 10 seconds, and have a portable warm oatmeal experience.
Yeah, it’s tough to make a bar or snack low sugar without sugar alcohols. Even the Larabars, which are just dates and nuts, have 15-20 grams of sugar in them.
Quaker has a lot of bars, judging from their product page. Any of these look familiar? Maybe the breakfast squares?
You’re welcome; I’m happy to help!
As others mentioned, you can find L&L stuff in lots of grocery stores including places like Whole Foods and Sprouts and also in sporting goods stores. Also Amazon (of course).
I’ll give those Munk Pack cookies a try; I like trying new stuff.
The squares are closest, but they’ve added filling, which wasn’t in the old style, and most of the flavors are new (I remember apple cinnamon, maple brown sugar, and I think oatmeal raisin). You might check out the nutrition profile and see if they’d work for you, I certainly remember them being good, and their ability to hold onto warmth was a huge plus.
I’ve come to like Oh Yeah! One Bars:
If you try them, I’d love to hear your thoughts about them.
Thanks, will do. 
Wow, all these bars I’ve never heard of. Thanks for the suggestion. It’s a little high in alcohol sugar for me, but I love the packaging.
Update: I tried the Munk Pack cookies based on the recommendation of Larry and Lenny Cookies by Snowboarder Bo (thank you!)
I tried the Oatmeal Raisin flavor of the Munk Pack cookies. I liked them. They’re very soft and very dense. If you’re looking for a regular fluffy cookie, this isn’t it. But I was looking at this as more of a protein bar and less of a dessert. It was slightly gritty, like the texture of fine oat powder or rice protein because both of those are in there. It’s slightly sweet, but not too sweet. The sweetness comes from raisins and date paste. It tasted very much like that.
I ate half a cookie when I was starving. It wasn’t a very good test because I was so hungry, but it kept the hunger away for a couple hours. I felt a sugar low later, but again, that’s because I was so hungry. On a normal day, this would probably work to keep me full for a while. Half a cookie has 9 grams of protein and 3 grams of fiber.
I also tried another flavor of Larabars. It was the Fruits and Greens Strawberry Spinach Cashew flavor. On the website the ingredients are listed in the exact opposite order as on the label of the bar. The first two ingredients are apples and apricots. That’s what I taste most. The spinach has no flavor and the strawberry is hidden. The cashews are flecks like in the other Larabars. It really should be called the apple apricot bar with a dash of strawberries and spinach. It was good though. The apples and apricots gives the bar its fiber content. And the rest gives it some nutrition. There are 3 grams of protein and 4 grams of fiber.
The sugar content on both of these is very high. If you’re looking for a low sugar alternative, these are definitely not it.
For nutrition - Protein + fiber - the Cliff Builder bars are tough to beat. I really like both the chocolate and the mint varieties. They had a chocolate hazelnut one that was close to being my perfect food, so naturally they stopped making it.
If you have a Costco membership then theirKirkland brand are worth a try as well.
If you want t avoid sugar, Glucerna makes a bar for diabetics that I think has sucralose (Splenda) in it. It’s a sugar alcohol, but it doesn’t have the explosive effects of malitol. I’m hypoglycemic, so I take them on plane and car trips. They only have 10g protein, which isn’t a lot, but it’s a start.
There’s something I tried once, and the one I tried was pretty good, but I don’t have a lot of experience with all the different flavors. Anyway, it’s called Wonderslim, and also is essentially sugar free, IIRC. It’s kosher, which is why I made a note of it. It has 15g protein, and a lot more vitamins than Glucerna bars. You can get them from Amazon.
Trader Joe’s carries these. For less than a buck apiece! I find it easy to get hooked on the Chocolate Brownie one; dense and rich.
My wife works with a nutritionist who swears by Larbars. Their fruit flavors (like Blueberry and Cherry Pie) are full of high-quality fruit.