Like traditional BBQ (e.g., low-and-slow cooking over coals) sous vide is most interesting when applied to meat, especially cheaper cuts that would normally have to get boiled, braised, or roasted to within an inch of inedibility before being soft enough to eat.

That being said, my absolute favorite thing to prepare sous vide is carrots. Lots of other root vegetables (turnips, burdock, sunchokes, new potatoes) can similarly be cooked in a water bath to a temperature just high enough to soften them without extracting flavor or nutrients via boiling, steaming, or sautéing. I've also had good luck with green beans, asparagus, and broccoli rabe.

Finally, if you're pescetarian seafood is also a great option. I'm not sure I'll ever cook lobster any other way after seeing what whole (shell-on) lobster tails are like cooked sous vide with a healthy dollop of butter.