I saw also another study where they were comparing people who train indoors (lifting weights in a gym) to people who walk outside, and found the same "shrinking hippocampus" with the people training only indoors, but they didn't make the link to spatial awareness. In fact, they concluded (IMHO wrong) that the reason for this is aerobic (walking outside) vs anaerobic (gym) exercises, but in order to make such a conclusion they should have had a third control group doing aerobic exercises indoors.
I personally prefer e.g. swimming (indoors) though, especially now when it has been rather cold in here. I swim 2km every Saturday, on top of MA exercises that's pretty much the aerobic exercise I get (and need, frankly). I guess my hippocampus is shrinking fast as well, not much spatial awareness needed in the pool. Maybe I should take a second job as a taxi driver.
I'd rather see a study that compared it against sports, particularly ones like basketball, baseball or tennis where there's a significant amount of spacial reasoning.
Personally, I like to play ping pong alternating between hands each round as it really helps my coordination out.