Running the trails.
Most cities have a park with walking trails that will go up and down and make things a little more interesting and challenging. If you are like me and aren't interested in a workout that doesn't kick your ass then you can sprint up every hill you encounter, for those of you that constantly stay in the aerobic training zones (<80% HRR) this will kick you into the anaerobic zone once in a while giving you new benefits from the workout. I had the "pleasure" (see; displeasure) of running in a park in grande prairie that had points spray painted on the trail, I was never sure what these numbers stood for and they didn't seem to have any rhyme or reason to them but sprinting from one then jogging to the next made it a challenge and a half since these points where not often evenly spaced, sometimes the sprint would last a loooong time. Anything can be used like this, you could jog any curved parts of the trail and sprint any short one or sprint on the curves that go left or right. Just for a few ideas.
IntervalsIntervals are a close relative to the hill sprints described above, both can deliver feelings of lethargy. Doing intervals are simple, you can use either distance or time to measure your interval and then run at a pace thats near the absolute maximum speed you can maintain for that set distance/time. Some distances are more aerobic in nature and some are more anaerobic in nature, so heres a quick breakdown of some of the more popular distances.
200m interval=29% aerobic and 71% anaerobic
400m interval=43% aerobic and 57% anaerobic
800m interval=66% aerobic and 34% anaerobic
1500m interval=84% aerobic and 16% anaerobic
Sprint PyramidsThis is one I made up myself. All you need is a baseball field and somethings to use as markers. Id use paces (3 foot steps) to measure my distances, feel free to use whatever means necessary. Set 1 pylon at the start, another at 25 paces, another at 50 paces, another at 75 and the last at 100. Id do 4 sprints to the first pylon (walking/jogging back to the start), 3 sprints to the next (walking/jogging back), 2 sprints to the next, and Id sprint to the last one and work my way backwards. It's short but you can also do more than one pyramid or a larger one, or add bodyweight exercises to the start or end of each sprint.
Well that's about it for now. I'm going to get off my computer and do something now.