There are a few ways.
On your calendar, click on an empty time slot, select Explore activities to add, and choose an activity. The software will add that activity to that slot.
If you have a large block of free time, you'll receive a notification about it. Click the notification icon and then the prompt for help. You can either navigate to your calendar to manually fill your free time or let the software recalculate your trip for that destination. If you want the software to recalculate, it will ask you to approve changes it plans to make.
The List view on the Day by Day tab will also show free time in your trip and prompt you to fill it. Clicking the prompt lets you explore activities to add to your trip.