Gratitude and Charms
Not every reward needs to have a game mechanic effect associated with it. As the DMG says on page 133:
Adventurers strive for many things, including glory, knowledge, and justice.
What is motivating the characters? Are they on a mission for their church? Are they trying to save their home town from danger? It is easy to overlook narrative rewards, but they can be more meaningful than mechanical ones.
Black Spike already mentioned rewards from authorities. In my experience, the honest gratitude of children and simple peasants can be an even greater reward for the players. It can validate what their characters are fighting to defend. People thank them, hold a celebration for them, or offer them free room and simple home-made food, or what modest gifts they can give in thankfulness.
Temporary charms or boons
If you are looking for something with a mechanical impact, you can protect yourself from accidentially giving out something that is too powerful by making it temporary or limit its number of uses. Blessings that enhance ability scores, armor class, weapons, or grant magic resistance are typically given by gods, so charms may be a better fit for an astral journey experience. The DMG says on page 228:
A charm is a minor supernatural gift, which can be received in a large variety of ways. (...) Some charms can be used only once, and others can be used a specific number of times before vanishing. If a charm lets you cast a spell, you are able to do so without spending a spell slot or providing any components (verbal, somatic, or material).
Give them something to reflect their experience, either from the list in the DMG, or make up your own, for example:
Charm of Heroism. This charm allows you to give yourself the benefit of a potion of heroism as an action. Once you do so, the charm vanishes from you.
Charm of Feather Falling. This charm grants you the benefits of a ring of feather falling. These benefits last for 10 days, after which the charm vanishes from you.
There are examples other charms and boons in The Wild Beyond the Witchlight, for example:
- Magic Boon: The boon's recipient gains a d4 and can, at any time within the next 24 hours, roll this die and add the number rolled to one ability check, attack roll, or saving throw made by it.