I know that pets and mounts won't be available in Sanctuary, but I was thinking about something like a worthless item related to the lore, or a physical modification that has no impact on stats. I could even be happy with a stone that says "Hi!" with Deckard Cain's voice every time it's clicked!
You can tell if an achievement has a reward with just a glance in the achievement window. The small star icon indicates the number of rewards the achievement has.
In the example below, the Scavenger achievement has 6 rewards because there are 8 levels of the achievement. All but the first and seventh level have rewards.