Spend an afternoon along the lakefront people watching and enjoying the lake view! Bring a blanket and some beverages for a relaxing afternoon! There are a number of trails you can take around the lake and kayaks available (https://www.lhadventureco.com/activities/kayaking)
There are a number of restaurants around Lake Hopatcong and the surrounding lakes!
There are a number of breweries and taprooms within 20 minutes of the hotel blocks!
A flat 2 mile hike along the lake with some nice views!
Mini Golf along the lake next to Alice's, a nice lake-front restaurant
New Jersey's only Western Theme Park! We haven't been, but it looks like it might be fun for kids!