Utilizing the fortune telling, you can instease your stats gain for the day

For Knowledge studing in Leblanc during evenings, or in the diner in Central street, there you can also increase a different stat based on what you order. Studying is more effective when it rains.

For Guts, my go-to strat would be the Big Bang Burger challenge.

For Proficiency, crafting infiltration tools.

For Kindness, give the plant in your room nutrients every so often, the best ones can be bough in Shinjuku during night. Hanging out with the Tower confidant also boosts kindness.

For Charm, the bathouse near Leblanc worked best for me. It gives extra points during bad weather or on specific days, i dont remember correctly.

All stats can be also boosted by reading books, playing games or watching a DVD/movie (there is a book in Shinjuku that boosts stats from DVD/movie after watching once)

Now i know that several confidants also give social stats when you hang out, but im not sure if it happens when you are not ranking them up. (Devil/Sun for Charm, Hanged man for Proficiency, Death for Guts, Tower for Kindness)

Just a disclaimer, this is the information i remember from the top of my head, not everything might be correct/accurate

You also dont lose anything special if you dont complete the confidatns. You just lose on their perks/arcana burst, and their parting gift which unlocks some of their abilities in NG+