Real foods that contain good stuff AND that my body will tolerate: 1/2 of a crunchy, natural peanut butter sandwich (I add sliced banana if I have); dried figs and apricots; assortment of nuts, always including almonds. Otherwise, I use all the stuff mentioned by others - GU, other gels, Cliff, Lara and other such bars, simple fruit and nut bars (co-ops carry), shot blocks (if it's hot) ... just depends on the length and the weather. Sat I am planning about a 45 mile ride, mostly trail, but the weather is supposed to be hot and humid. So, no pb sandwich (gets too runny in the heat), instead a fruit and nut protein bar a packet of gel, couple of shot blocks and my normal bottle of water (I'll be able to refill) and bottle of partially diluted gatorade - full strength is too sweet for me. If all goes accordingly, I likely won't eat one of either the gel or shot blocks.
Forgot to mention - on long, hot days, nothing is better than Pearson's Salted Nut Rolls!
2001 Trek 7500 FX, converted to a hauler - Serfas
200? Marin hybrid - Selle San Marco
2004 Trek 5200 - Avatar
2011 Trek 6.2 Madone - Ruby