I love the straight line with the barrels thing. I use it as a progressive game with Nico...trot to the first one, come around that barrel in a trot and cue a big lope, canter around and cue a gallop...ride that last one like your last barrel and then ask to bring it down to a lope...and then to a trot.
I love when my horse is totally focused, best way to get that is to throw some totally unexpected things his way. Tons of transitions get him up and under himself. Sudden turns (to him, not me!) are a great way to get his focus.
Like sorrel, I love using 'spin and runs' as a tool. Instead of "hey" I tell him 'whoa-turn'...i started doing this in the round pen and its actually a lot of fun. Get him going on a nice flat stretch...pick a spot and when you reach it lay your heel on him and say 'hey' or 'whoa-turn!' and expect a 180 turn and a lope off.
Long sloping canters are great for stamina and are a great way to relax...