A rule of thumb to remember is:
Short burst of high intensity activity -> like lifting a heavy weight for 1-3 repetitions will have little to no effect on your blood glucose.
Longer burst of moderate to high intensity -> lifting weights for +8 repetitions and several sets (think of a bodybuilding routine, or what most people do in a gym to "tone") or moderate intensity cardio (something you can do for extended periods like 30-60 minutes) will tend to raise your blood sugar.
Long moderate activity -> longer than 60 minutes which is usually moderate intensity throughout tends to give a rise and a drop and could require some form of carbs or sugars to continue.
Something else to keep in mind is that your body conceives exercising as stress triggering a release of cortisol. Long story short this can reduce insulin sensitivity in the short term. As your body becomes fitter this stress response will become less but in any activity you try to improve yourself so chances are that your will increase the intensity and this stress response will remain. It's a nuisance but the overall benefits of exercising outweigh the short term negative effects from it.