What to feed pet rats...

Many rat owners on various forums recommend Suebee's Diet. You could also try Melissa's Rat Mix.
We make a mix for our ratties using either Rat/Mouse VitaMix or Reggie's Rat food mixed with Total cereal, Kellogg's Cornflakes cereal, granola cereal, pumpkin seeds (roasted, unsalted), and organic dry rolled oats. We also give them Harlan blocks every day (which most of them promptly store somewhere in their cages). Our ratties love treats like fresh carrots, peas (we put frozen peas in a shallow dish of water - the ratties love 'fishing' for the peas), green beans, cooked Quaker Maple and Brown Sugar oatmeal (we are, of course, careful to make sure the oatmeal has cooled off before feeding it to them), mashed potatoes (no salt), yogurt (raspberry seems to be their favorite), yogurt drops, low-fat dog biscuits, bananas, Earth's Best Organic Oatmeal Cinnamon toddler cookies, Gerber toddler meat sticks (they'll each get half a meat stick, never a whole one), chocolate chip cookies (we break up a cookie into 6-8 pieces so they each get a small piece) - chocolate will not harm your rat but it should not be a regular part of their diet simply because the fat, sugar, and caffeine really aren't good for them (but a small amount of dark chocolate can help a wheezy rat breathe better because it helps open up the respiratory passages).
For sick ratties we give Ensure, baby food, and just about anything else that we can get them to eat to help them keep up their strength. When medicating we first find what food appeals to the sick rattie and then mix the medication in with a tiny portion of the food or Ensure.
This should go without saying but always make sure your ratties have fresh water. It's a good idea to check the water bottles every day to make sure your ratties are able to get the water out of the bottles (we purchased a couple of new water bottles and quickly discovered the water wasn't being dispensed like it should have been so our ratties weren't able to get any water).
NEVER feed your ratties raw sweet potatoes, blue cheese, raw beans, raw nuts (most nuts found in stores are cooked/roasted), green bananas (ripe bananas are fine), peanut butter, or dried corn.