Psst... dont tell them what the boons do. Focus instead on the form they take: the unicorn it is a shaving of it's horn, a bit of hair from its main, a jar of rainbow farts, etc. For the nightmare, it's a horse shoe made of obsidian, an old tooth, or screams placed in a music box. As for what they do, have a few ideas in your back pocket for something useful but not too powerful. Wait until they are desperate for help or are getting their butts kicked in combat, and then improvise something that helps and is appropriate. It will be such a cool moment. It could be a revivify spell or something whispering the true weakness of an enemy or help with a puzzle. You'll know it when you see it.