The Starfield Mantis puzzle might be the most important early game side-quest, but it's astonishingly easy to miss.
This article contains minor Starfield spoilers.
It's easy to pick up the Starfield Mantis quest, but you might not even know you've done it. Random Spacers will drop a slate labeled 'Secret Outpost!' that points you toward a planet called Denebola I-b. Once you've picked up that slate, you'll have unlocked the quest, and if you're anything like me, it's been sitting there ever since.
One of my colleagues, however, told me that it's definitely worth chasing up, and now I'm passing on the favor to you. Once you land on Denebola I-b, you'll be greeted with a gauntlet of spacers trying to make their way into a secret lair - that lair is well-defended enough that you probably won't want to take a run at it straight away, but I managed to clumsily make my way through it somewhere around the level 10 mark.
The reward for getting there - assuming you can beat the Mantis puzzle that stands in your way - is definitely worth it. As well as some legendary gear (armor, a helmet, and a pack), you'll also be able to claim a brand-new ship that's a great first upgrade from your initial one. It turns out that 'The Mantis' is some kind of Starfield superhero, and while they were trying to pass over that mantle to their estranged son, you've managed to scoop in and claim the prize for yourself.
That armor offers a host of nice boosts, and the ship is definitely what you need to kickstart your Starfield ship customization. Whether you'll want to use it by going around the galaxy and putting the fear of God into Space Pirates is up to you - I feel like there's a certain poetry in using this stolen superhero ship to do Space Evil - but however you choose to use the mantle of the Mantis, you should definitely do so sooner rather than later.
If you want another early advantage, some Starfield trickery will get you an excellent armor set.