I like GU gel (peanut butter chocolate, salted caramel or espresso love are my favorite flavors) or Hammer gel (hazelnut all the way!) Check the nutritional info on the pack, some gels have caffeine, some do not, some have more electrolytes than others. Gels are ~100 calories each, and I'll suck one back on training runs between 13km-21km. For races, i'll sometimes grab a gel for 10km depending on the time of day and when i ate/had coffee, for the half i ran, i had 2 gels.
The Clif shotbloks are tasty, like candy! They are ~30 calories each if i recall correctly. I'll usually pop one of them when i get home from work if i need a quick pick me up before a short run. I always buy the tropical flavor which has a bit of caffeine in it too.
Practice your fueling in your training runs. I notice that both my husband and I slow down when we're sucking back our Gu, so, I'd opt for a higher calorie fuel less frequently, especially if you have issues with chewing while running.