Marvel's Spider-Man is a quite long game and full of additional activities. During the break from rescuing New York City you can catch pigeons, solve Black Cat puzzles, take photos of wonderful landmarks and prevent minor crimes. Despite that, a question arises - is it possible to go back to side missions, after completing the main plot?
The answer is yes - once you've completed your main missions, you'll be able to return to all side activities. So if you want to leave other missions for later and focus on the main storyline - nothing stands in your way. However, note that side activities are unlocked within time - at the end of the game there will be so many of them that you may feel overwhelmed.
It's especially worth focusing on stopping crimes and cleaning up criminal bases. These missions are quite long and repetitive - so perform them in a regular basis. Additionally, once you have completed the main plot, you will be able to change the time of day in research stations.