GoogleMaps is a great tool to make a custom itinerary for several reasons. You can plan your trip efficiently by grouping activities together on your itinerary that are close in proximity on the map.
It’s a good idea to first break your map into categories before adding pins. Ideas include food and drink (or further broken down into restaurants, cafes, bars), accomodations, sites and activities, shopping, essentials (grocery stores, pharmacies) and more. You can then create a separate map layer for each category.
For each activity, estimate the length of time it will take to get there, and how long you will spend in the location. For each pin on your map you can see additional location information such as a website, phone number, address, reviews, and website when you add it to your custom map. You can also add your own extra details when planning in the notes of each pin.
You can then plan your driving route by creating a layer with directions. This way you know how much time and gas money each leg of the trip will be. Other customizations like categories, color coding, icons, etc give you the ability to personalize your map and make it work best for your needs.
If you are trip planning for a group, and trying to cater to the desires of the masses, you can easily do so by sharing your custom map with friends and giving them edit access to make it collaborative.