A ton of stuff!

• Potion of cleansing (exotic purity) removes all debuffs including hunger/starvation. This means potions of purity become a major food source when On Diet is turned on.
• Elixir of aquatic rejuvenation gives more healing than a regular healing potion and allows you to control when and how much healing to apply (by standing in water) to let you avoid wasting healing at full health. This is crucial with On Diet + Barren Land because food is a scarce and precious form of healing, dew drops and sungrass are out of the picture, so you don’t want to be sitting at full health unless you’re starving.
• Elixir of honeyed healing is another source of nutrition.
• Elixir of dragon’s breath makes you immune to fire and imbues your attacks with fire. This is extremely useful against the fire hand and the grass hand in the final boss fight.
• Elixir of toxic essence makes you immune to poison gas and causes you to constantly emit toxic gas. This protects you against the toxic gas hand and also gives you more damage against some of the other hands.
• Transforming unidentified scrolls into runestones in order to identify the scrolls. Once you have scrolls of upgrade and remove curse safely identified it’s generally a good idea to turn all unidentified scrolls into runestones. Many of the scrolls are actually more useful in runestone form (especially rage and teleport) and getting all scrolls identified quickly gives you more options.
• Upgrading unidentified potions into their exotic form. Once you have potions of healing identified (which you can do by observing flies/necromancers/vampire bats drop them) you are safe to turn all unidentified potions into their exotic form. This allows you to identify potions by drinking them in exotic form which is much safer to do. The only dangerous exotic potion is corrosive gas which you can mitigate by standing diagonal to a door and running out of the room to escape the gas (which will stop spreading when the door closes). If you wait until you’re starving and moderately low health to identify your unknown exotic potions then you won’t waste a potion of cleansing should you happen to have one.
• Aqua blast spell (crafted starting with a potion of levitation) gives you 8 charges of a spell that lets you create pools of water anywhere. This is extremely useful for getting rid of traps. 2 casts will clear the toxic gas vents in the treasure room, letting you loot a bunch of gold without spending your potion of purity (which is much more valuable as a food source). Aqua blast is also great against blazing champions, fire elementals, and the fire hand.
• Feather fall spell (also from levitation) gives you 2 charges that let you loot the drop-down rooms without spending any healing or food resources.
• Beacons of returning (crafted from scrolls of teleport). You get 5 of these which are basically get out of jail free cards that will save your life. They are also great for backtracking to a shop without spending tons of food running there. Even more powerful is to combine one of these with a scroll of passage (exotic teleport). First you save the current location (using the beacon), then scroll of passage to get back to the shop, then use the beacon to return to where you started. An expensive use of scrolls but saves a ton of food. Very powerful use for the Unstable Spellbook (which can basically give you a free scroll of passage) but you need to be prepared for it.
• Potion of Divine Inspiration (exotic experience) gives you 2 bonus talent points in a tier of your choice. Extremely good! From T2 onward you generally want as many talents as you can get!
• Scroll of Metamorphosis (exotic transmutation) lets you swap out any talent for a random talent of the same tier for another class. Very good for swapping out situational or obsolete talents (extra food or identify faster T1 talents) for something more useful!

There’s far more stuff than all this. Pretty much everything in alchemy is useful if you’re creative enough about it. This is really important as you add more and more challenges because resources become scarce, damage you take increases (a lot), and a lot of easy strategies are disabled.