I love the Simple Great Battles of History rules. It is one of my most played games. The only thing I was never quite happy with was the activation rules which I felt allowed play to over focus on one part of the battle too often. I wrote these house rules back in 2012 and they are posted on the superb support website of Brendan Clark: http://www.gboh.net/6.html
Do check that website out if you are in to Simple Great Battles of History. These rules are an improvement on my 2009 attempt.
If you want to look at another excellent option for modifying the activation rules then I suggest you also get a copy of GMT's C3i Magazine issue Nr 26. In this issue Dan Fournie provides an insert offering Magna Graecia scenarios. In it he also offers yet another take on an activation system for Simple Great Battles of History that I feel can be applied to many other battles in the system. But given the range of battles in existence now it is hard to retro-fit a rule that applies to all scenarios covered by the system. I feel I achieved it with more of a tilt to the original Great Battles of History leader activation system. However, this also makes the rule more complex. Dan's approach tilts more toward Simple Great Battles and therefore the rule is simpler - and I heartily recommend it!