Carrots are good in limited quantities but they are high in sugar and low in vit c. Cheerios might be ok as a treat but I'm not sure they provide all the nutrtion needed for pigs. Alfalfa hay is also ok in small amounts but if you are on an alfalfa based pellet grass hay is best as alfalfa hay is high in protien and too much isn't that great. If she likes basil that would be a good treat in small amounts too. I personally avoid petstore treats as they are usually full of junk and sugar veggies and fruits are healthy and they love them!
I am 40 and have had guinea pigs since I was about 12. When we first started out, I used the guinea pig pellets as main diet and my guinea pig had a vitamen deficency. Our vet recommended Cheerios and all my guinea pigs have been on that ever since with no issues at all and lived VERY long lives (my friends used to comment about how they are "STILL alive after all this time!?".
Also, for care requirements, make sure guinea pig has hard things to chew on to keep teeth filed down and you need to make sure claws are trimmed regularly.