From http://www.squidoo.com/guinea-pig-care-4 ;

Here is a list of some of the fruits and vegetables that guinea pigs
particularly love and that give them the best nutritional benefits. Fruits are
high in sugar so it's best to view them more as treats, and avoid any sudden
major changes in diet until you know how your guinea pig's tummy reacts to
things:

• Red Pepper (or green/yellow pepper. Red has the highest vitamin C
  concentration)


• Carrots - both the root and the green tops are perfectly safe for your
  guinea pig.


• Apples - just make sure they don't have any seeds.


• Oranges


• Leafy greens like red lettuce, romaine, Boston lettuce, (be sure to feed
  leafy vegetables in moderation to avoid diarrhea, and feed spinach a bit
  sparingly to avoid potential kidney problems.)


• Cucumbers


• Broccoli (in moderation since it can be gassy and cause a gas pain)


• Green beans


• Grapes


• Bananas


• Dandelion Greens


• strawberries (in moderation)


• oranges or clementines. A helpful way to feed these to your piggie if
  they're being picky is to scoop out some pulp from half an orange and leave some
  in it so they can lick it without needing to chew on it if they're not diggin'
  the texture.


• spinach


• cherry tomatoes in moderation


• cilantro


• dandelion greens


• parsley - either curly or flat. They
  love it and it smells delicious when you're tearing some sprigs off the bunch to
  give it to them!

             Foods to NEVER feed your Guinea Pig,
EVER!         
what not to feed your guinea pig, no matter how good
it may taste to you.
While guinea pigs do enjoy a variety of flavors
and types of fruits and vegetables, there are some foods that you should never
feed to your guinea pig. If there is an item that isn't on this list or the one
above and you're not sure, it's best not to feed it to your piggie until you can
get verification from a vet. The good foods list isn't comprehensive, so you may
find some other online resources with more exhaustive lists.

The
following is a list of food items to NEVER feed to your guinea pig.

• Iceberg lettuce - this won't kill your guinea pig if you give him or her a
  little piece, but it has virtually no nutritional value, is quite fibrous and
  watery. It's been known to give guinea pigs diarrhea. With the great selection
  of other more leafy, dark lettuces available, just avoid this one and go with
  one of the others.


• Any type of cabbage - like iceberg lettuce, a small piece is not likely to
  severely harm or kill your guinea pig, but it's very gassy and not good for
  their little systems. Stay away from it if you can.


• Cauliflower - also very gas-producing.


• Potato peelings - not that this would be something you'd be likely to give
  your piggie anyway, but toxins can exist in potato skins, particularly if
  they've begun to turn green, and they can be poisonous. Just toss them in your
  garbage disposal or trash instead of your piggie.


• Raw beans (this doesn't mean green beans. This means things like raw, hard
  kidney beans, split peas, pinto beans, etc.)


• Shelled nuts or seeds, particularly things like sunflower seeds or 'bird
  seed' type mixes, pumpkin seeds, peanuts, etc.


• Rhubarb - the leaves of the rhubarb plant are poisonous, and the stalks
  are very, very sour.


• Any type of meat or fish. Guinea pigs are vegetarian and looove a salad
  over a steak any day.


• Dairy Products - while many stores will offer 'yogurt drops' and other
  'treats', dairy is not healthy to feed your guinea pig and can actually cause
  problems. Feed natural treats like veggies instead of store brand treats. It's
  less expensive and your piggie will be much happier.


• Chocolate, coffee or other caffeine-containing products.


• Alcohol (this should go without saying!!)


• Cookies/bread*
  
  *you can feed a small piece of cereal like a cheerio
  from time to time as a treat, but they will likely not even like it, so it's
  best to just avoid this altogether!


• hamster food, gerbil food, rabbit food, or any other non-guinea pig food.
  Often these contain seeds, which are a big no-no, but they also have a different
  balance of vitamins and minerals which aren't suited for a guinea pig's dietary
  needs.


• corn kernels. Popcorn isn't a very good idea either. Guinea pigs choke
  easily and think of how easy it is for us humans to get popcorn kernels stuck in
  our mouths and throats!


• sugary foods like donuts, candy, or soda


• Ice cream.


• Any processed or fried foods that you would eat, including potato
  chips/nachos or french fries.


• Cheese


• anything spicy. jalapeno peppers, anything with cinnamon,
  etc.