So far, the kinds of boons I've offered have typically been something thematic that provides additional options (because who doesn't like options?).
While an extra spell known is nice, you still can only cast so many spells. An additional invocation would be good. I've also given a player an additional cantrip.
But the best boons are (in my opinion) created from scratch and relevant to the one providing the boon and the favor that was done to earn it. So what did the player's character do to earn his patron's favor?
A couple of ideas that are coming just off the top of my head.
Bend Space
As a bonus action on your turn, you can teleport a distance up to your movement. You must be able to see the spot you wish to teleport, and it must not be currently unoccupied. You can use this a number of times equal to your proficiency bonus. Afterwards, you cannot use this ability again until you have completed a short or long rest.
Here is a boon that I gave to a wild magic sorcerer in my current campaign:
Warp Reality
Once per long rest, when you successfully affect someone with a spell you cast, you can choose to curse the target make space warp around that target similarly to a Gibbering Mouther. The target experiences the area immediately around them become dough-like and difficult to move in. Their speed is reduced by half and they can't take reactions. This is a permanent effect on the target.
Secret Whispers
You have gained the ability to psychically taunt your opponents with their greatest shame and guilt. On your turn as a bonus action, you can target one creature within 30 feet. That creature then begins to hear whispers and see ghostly images flash before their mind's eye that causes them anguish. This creature takes psychic damage equal to your proficiency bonus. There is no save and no attack roll required to use this ability. The target must have at least animal level intelligence to be affected. Using this ability does not grant you knowledge as to what the target sees or hears, but brings it forth in their mind in a manner most uncomfortable.