The first thing you need: water, of course! Especially while running, water is the thing I like best. It puts no stress on your stomach, and the only risk is that you might get stitches when you drink too much in one sitting. When it is extremely hot, I like to add a little salt to it—with all the electrolytes you lose through sweating, I really have the feeling it helps.
While running, I hit the wall only once—while on a 30+k run at freezing -5°C during the foolish days of my youth. When cycling, this happens much more frequently (you may remember the cherries that saved me; I had similar experiences with blackberries and other stuff that grows next to the road). I have the feeling that your reserves deplete much more sneakily while you cycle, so I often take some carbs with me, just in case. Recently, I got a free marathon pack, which included many tasty things (plus a water bottle). Not that I’d be capable of running a marathon, but they did not ask about it, so I got the package nevertheless 😉 Currently, I am trying out all that funny stuff. Here’s a picture of the two samples I tasted recently:
The stuff on the right-hand side is a classical energy gel. If you are not used to using energy gels, it tastes a little weird. I would have expected something in between Milkmaid (the concentrated sweetened milk you get in cans) and dissolved gummy bears, but it was totally different. First, it was less sweet and less sour than I expected; the taste is rather starchy, with a pronounced note of artificial cherries. However, the effect was noticeable: when I was running low on energy, it really helped me recover. (Maybe it was a mere placebo effect, but who cares as long as it works ;-))
The powder on the left-hand side must be dissolved in water before use. The peach-and-passionfruit taste is very refreshing and sour, but although I have a sensitive stomach, it did not cause any pain or heartburn at all, at least not while cycling. Delicious. Lucky I have one bag left 😉