Every alchemist has their favorite formula. Pick you own, or determine yours using the table below.

During a long or short rest, you can collect materials for 16 gp. You can then experiment with the materials. At the end of the rest, roll a d4:

1. It doesn't work
2. It can be used to create a sensory affect, like a puff of colored smoke, bubbles, or a terrible smell, which is of the DM's choice
3.  It can be used to attack, by throwing a vial of acid or alchemist's fire, which is determined by the DM, but you are considered proficient with it, it is a Dexterity attack, and it deals 1d6 damage 
4. When drank, it restores 1d20 HP

If you roll a number other than 1, you gain 1d6 vials or containers of the substance. If you manage to make six of the formula, you can write down the recipe in your formula notebook. Then, the next time you make this formula, you only spend 10 gp, and can make 1d8 vials or containers of the substance.

Instead of an Alchemist, you might have been a Potioneer, who made potions, rather than alchemical formulas. Instead of getting proficiency with Alchemist's Supplies, you could get proficiency with the Herbalism kit, which you could also get instead of Alchemist's Supplies.

During a Long rest, you can spend 1 hour and 20 gp worth materials and components to craft a Potion of Healing or a Potion of Climbing.