I LOVE summertime! What a beautiful time of year to put those strong muscles (that you've been strength training all winter) to work doing something fun like outdoor aerobic activities. Get out and go for a day hike, head out on a mountain biking trail, or just take a morning walk or swim before the day gets too busy or hot! It will make you feel great.
Remember, if swimming or road biking is your main heart healthy exercise, also get some weight bearing aerobic exercise for the stimulation to your bones. Some research has shown that the bumpiness of riding a bike off road is beneficial to bones, so take a ride on the trails and enjoy the scenery!
I hiked this morning on a path near my parent's home in Bellevue, Washington. It is a 2nd growth forest in the midst of suburbia that was preserved from development. What a treat! It goes on for several miles, following the Lake Sammamish road.