Sloppy Joes -- loose ground meat cooked in a sweet-tart tomato sauce, then piled on a bun -- are almost but not quite a meal in themselves. Side dishes should match the casual feel of the meal and not be too fussy.
Cold "picnic salads" such as coleslaw, macaroni salad and potato salad are natural partners to sloppy Joes. Either mayonnaise-based dressings or vinegar-based dressings can be used to adorn the salads; use your favorite preparation or feel free to experiment with add-in ingredients and nonstandard recipes.
A large lettuce-based tossed salad is another good option to pair with sloppy Joes. You can make this side dish as simple as a tangle of baby greens dressed with a vinaigrette, or as complex as a fully loaded chef's salad complete with creamy dressing. A crisp romaine lettuce is particularly nice here, as it plays off the soft texture of the sloppy Joe well. You can also add crispy and crunchy ingredients such as croutons, sunflower seeds, nuts and raw carrots and celery to a softer lettuce salad for the same effect.
Other Vegetable Sides
Corn on the cob's sweet flavor matches well with the sweet and tangy sloppy Joe sauce, so definitely consider it as a side dish. Sauteed green beans are more subdued in flavor, but if cooked to maintain a bit of their crunch they provide a pleasant textural contrast to the soft and meaty character of a sloppy Joe. Steamed cauliflower or broccoli, dressed with sesame oil, brown butter or olive oil and lemon, is another good option. Some pickled vegetables add a nice tang to contrast with the sweetness of the sloppy Joe sauce. In a pinch, raw carrot sticks can be served -- kids in particular often like this option, and it's easy to boot.