Hello Paul,
The “Activity Levels” are “radio buttons” in backend because only 1 activity level can be assigned to a tour, so they can be either “easy” or “tough”. However, in the frontend, we have icon based filter which will allow users to chose one or multiple activity levels, like they can filter results to show either only the “easy” or both “easy” and “tough” tours.
Hope that clarifies your query. As for the customization, like editing the backend to replace radios with checkboxes, you can use theme / plugin customization services like WP Kraken since they have already customized our theme for many users.
Thank you