Hello Cindy775,
Field Service is an additional app available on our Method Classic platform, therefore we welcome all questions and concerns about field services on our Method Classic general Q&A section.
Coming back to your question, you can easily set both activities and work orders to show on one calendar. Let's say you want to use Activity Calendar, the only thing you need to do is change "App Specific" setting on WorkOrder type.
You can achieve that by going to New_Activity screen, Click on three dots button on Type drop down. It will open up a new screen(check screenshot) and now select WorkOrder activity type from the list and uncheck the App Specific option and save it. Once it is unchecked and it's value is showing as No in the grid, WorkOrders won't be restricted to Field Services app only, it will start showing up on you Activity calendar also.
I Hope that helps!
-Inder