If you dip apples in lemon juice, they will not go brown. I used to do that for soccer snacks for the entire teams my kids were on all the time and it works. I put lemon juice in a bowl, cut the apples, put them in a bowl, then set them on a towel to dry.
Grapes are great and priced lovely right now.
Strawberries, orange slices, peaches, kiwi, mangos, plums.
My kids love fruit. I would not put sugar, there is enough natural sugar in them. Also, my 21 month old eats grapes just fine without peeling or slicing, it is actually his favorite fruit. I taught him to chew, he asks for those and plain strawberries all the time!
Just keep trying new things. Most fruits are cheaper than buy junky pre-packaged snacks and way more filling to the tummy. Buy a couple new things at a time and see how it goes. You could even get the frozen kinds, we do that in the off season and they enjoy them just as much. :)