This is one thing they're very strict about on the dining plan - no trading desserts for appetizers at TS. You can ask if you can substitute fruit or sorbet or something if you really don't want the dessert but still want to order something, but you can't get an appetizer.
At CS, you may be able to trade the dessert for whatever they're willing to give you, maybe a bottle of water, maybe some fruit, maybe a bag of chips. A side dish would be less likely.