Tea

Tea leaves are popular for making a hot drink, whether for medicinal purposes or enjoyment. It's mentioned many times in the film franchise.

Description
Tea is traditionally prepared from the dried leaves of a shrub native to Asia the Camellia sinensis order, when they are infused in hot water to create an aromatic yet bitter drink. However many other plant leaves from all over the world and even spices can be made into teas as well. Today, teas come is thousands of different flavors originating from different plant/spice combinations.

Function
In the franchise, teas are usually seen only for medicinal use, though they are mentioned for their leisure enjoyment use a few times. In real life tea is used for both as well, and a staple drink commonly associated with high society in England and around Europe, and a daily companion to meals in Asia.

How to Train Your Dragon
Though not seen or even mentioned directly, one of the songs from the movie soundtrack is titled "The Vikings Take their Tea".

Dragons: Race to the Edge
In "When Darkness Falls", Tuff says that yak dung can be made into a pleasant aromatic tea, and Ruffnut agrees, suggesting they've both had it before.

In "Reign of Fireworms", Tuff again brings up "Yak Dung Tea" and suggests giving it to the other Riders when they are thrown into the dungeon for disobeying the rules. He makes it sound like it's disgusting rather than pleasant, and thus makes it part of the disciplinary action.

In "Snotlout's Angels", the Wingmaidens give Snotlout a tea that is suppose to help him regain his strength. He loves it, and slobbers it all down his face while drinking it, after which he asks for more.

Games
In the game School of Dragons, both Astrid and her mother have the player make medicinal tea to help with illness and insomnia. Thornado also needs tea for an upset stomach, and Hiccup even makes tea for Astrid who is sick.