I always use gel flasks, I just make sure to lick the nozzle off as good as I can before stuffing it back in my jersey pocket. The only time I had a problem was when I didn't click an EFS Liquidshot flask shut, that was a major mess but it all came out in the wash.
For energy bars (e.g. Cliff) I pre-open the pouches, when I'm done I just stuff the empties in a spare jersey pocket. They don't take up any room when empty.
As far as drink mixes go, you just have to experiment to find one that suits your taste. Try stopping in a bike store and picking up an assortment of the drink mix pouches.
If you're looking to stay perfectly clean with no icky-sticky on your fingers from gels, you may need to reconsider. Part of the fun of riding is getting a little (or a lot) grimy, that's what the post-ride shower is for. Just relax and enjoy the ride.