I can't be certain that all of these ideas are allowed on South Beach. I have an endocrine disorder and so follow a very low-carb meal plan because my body doesn't like it when I eat too many carbs. My doctor & I put this together, and we stole pieces from various published diets.
Also, until this year, I didn't have access to a fridge or microwave because I teach. We didn't have a faculty eating area in my last school, and the only available refrigerators were the ones that the students were allowed to use. That caused way too many missing lunches for my tastes.
The ice-pack in the insulated bag works very well, but so does some of these other things, as long as you use an insultated bag or ice-chest (they make little ones).
Frozen pre-cooked shrimp, tail on or off... they thaw by lunch and also act as a tiny freezer pack
Sliced raw cucumber
Pea Pods (not the sweet peas themselves, but the pea pods like in oriental cooking)
Low-fat cheese chunks (I don't buy anything above 2% milk... Cabot makes a really good cheddar, and the Cracker Barrel cheese is passable; I've found awesome mozzarella balls and smoked cheeses at Trader Joes and Whole Foods)
Canned tuna (I use the white albacore, because that is a preference, but you can use any; I also buy the 3 ounce little cans... its a bit more expensive unless you go to a wholesale club, but the convenience is worth it. The small cans have flip-tops, and its portion controlled. One can is between 80-100 calories)
Fat-free sour cream mixed with some powdered dressing of your choice (to mix with the tuna or dip the veggies in. I usually use ranch.)
Roasted turkey breast (a local grocery store roasts in-house with all kinds of flavors. You should be okay as long as you stay away from any that say 'honey' or are sweet) sliced. I usually take a bit of miracle whip (low fat), lettuce leaves and cucumbers and wrap it together.
Roast beef (same as above, though I often put my cheese choice in this wrap)
I should note that you can substitute canned or roasted chicken for most of these. I don't, because I can't eat chicken... it is one of the foods that I can't tolerate anymore. Which really bites, because I used to LOVE chicken!!! And, because chicken is used in just about everything!
I was also allowed to have berries and low-fat cottage cheese when I started this, so I'm not sure if those are allowed on South Beach or not.