Watch the video for visuals to help with the guide below:

Gems are used to upgrade the house and underworld. The first tab contains things you should focus on like adding rest areas with fountains, making some pots contain money, and even the ability to unlock the administration chamber which is important for completing a few quests. The other tabs are just cosmetics.

The first time you give someone nectar they will give you a keepsake which you can equip wherever you find the storage container. [Switching keepsakes during a run will lock the container and you will need to find the next one to switch again]. Keep doling out nectar and eventually characters will accept some ambrosia which further unlocks your relationship status with them.
There are even a couple romances you can have and I just wanted to say there are no repercussions for liking more than one person so sleep around if you like.

If you’re interested in gaining all the Companions which allow you to summon an ally during an encounter, then focus on giving nectars to:
Megaera, Thanatos, Skelly, Sisyphus, Dusa, and Achilles. Each companion is good in it’s own way like since Skelly creates a dummy which taunts enemies into fighting him, he can be great in the Erebus portals where you have to complete the trial without taking damage and those rooms also give you better rewards. Personally I leveled up the Sisyphus companion first since it drops a boulder which damages a nearby foe and also drops food, darkness and money!
[You cannot use all the companions against the end boss until you unlock a few hearts with Hades]
Speaking of Sisyphus, once you reach a certain relationship level with him he will introduce you to Boldly and allow you to give it nectar! This will give you a minor random buff that you can see in your list of Boons.

All 6 weapons allow you to use titans blood to upgrade and unlock new aspects once you find them. Each weapon's 4 aspects can be upgraded separately meaning you will need a lot of titan blood.
If you’re worried about spending this precious resource you can try this; go ahead and upgrade a weapons aspect with some blood or a companion with a bunch of ambrosia, then take them out for a test run. If you don’t like what you see, simply pause the game and give up to reset back to the courtyard. This returns all of the resources you spent and reverts the companion or weapon to it’s previous level. [PRO TIP: This also saves the runs seed meaning your first boon will always be the same!]

Titan blood and ambrosia may seem scarce but remember that you can trade excess resources at the resource broker and you can win 2 titan bloods, one diamond, and one ambrosia by defeating the various bosses each run with each weapon and these rewards reset for each Heat Level increase you apply later on. Also, the chef will reward you with some of these items depending on the fish you catch.

When you unlock fishing, look for these sparkles in the rivers or an audible chime/ding:
Each area has different fish with unique rarity and rewards. Your goal is to catch the fish when the bob becomes fully submerged. The closer your press the button to this point, the better your chances of finding a rare or legendary fish.
However, the fish may try to trick you and tug a bit without submerging the bobber. They might try this up to a maximum of 3 times, after that, the 4th pull will always be a fully submerged bobber. Thus, do your best to time your catch if the fish grabs the bait early but make sure your finger is on a hair trigger after three fakes since you know the next tug is going to be the big one.

Back at home you might have noticed the fated prophecies stating that Zagreus will learn to play the lyre. To earn this, first purchase the lyre from the contractor then make sure you pluck a few strings any time you're in your room, or sit there and interact with it for a while. Eventually (and this will take a long time) it will unlock conversations with Orpheus and ultimately complete the fated prophecy.

Each upgrade you unlock in the mirror has an alternative upgrade which can be swapped by pressing this little button on the side. Check both abilities on offer to see which you prefer since you can only use one or the other and they upgrade independent of each other.

One day you might be visiting Charon’s shop when you see that this cheeky bugger has an entire bag of gold just sitting there... [taunting you]. Collecting this coin will automatically transfer you to a secret arena where Charon himself will challenge you to a duel. Beating him not only lets you keep the coin you stole but also gives you a special discount card for everything he sells later on (for that run). JUST remember that grabbing the money will transfer you right away so make sure to purchase what you want before you pilfer his stash.
Also, the last Charon shop sometimes has an anvil which offers upgrades for your weapon. Normally you can only have 2 Daedalus upgrades per run, however, the anvil will pick one of your upgrades and replace it with 2 other randomly selected upgrades allowing you to play the end of the game with 3 upgrades. Since you have no choice in what upgrades replace the one you lose, it can really change your playstyle so be flexible if you opt for this upgrade.

When you complete a run you’re presented with this score screen which gives you some stats and information but also the ability to select and deselect boons and upgrades. This simply allows you to showcase certain upgrades if you … say … want to brag to others about your ending build. Normally I just click past this and pay little attention to it.

Bonus video for fun!