I would rather my children eat very moderate amounts of real sugar than artificial sweetener.
You can find bars of dark chocolate (70% cocoa, etc.,) Which have very little sugar. You can even find natural/organic chocolate which has no artificial ingredients. They're very rich so a tiny bit satisfies a chocolate craving.
Other healthy substitutions could be a few raisins, or a bite of fruit or cheese or a spoonful of peanut butter. My kids are big fans of frozen fruit - especially frozen grapes. I like the smoothie idea, if you blend fruit and/or juices or milk with ice it really does taste like a milk shake or slushie. I like to use (unsweetened) coconut milk because it has lots of healthy efa's. I often freeze fruit in chunks then blend with a small amount of liquid to make a "sorbet'. My kids love it - they think it is ice cream ;-)
Popcorn is another favorite snack around here, so are plain salted tortilla chips - both are whole grain. Watch for trans fats in any prepackaged snacks (most are dropping them now...)