Go with the flow.
Don't plan out EVERYTHING! Summer is the time of freedom, and when you are less stressed from knowing absolutely everything in advance, you will be much more relaxed.
Go on adventures.
Take a road trip somewhere new, stop and take pictures along the way. Go on hikes, go to festivals, indulge in new experiences that you will remember for the rest of the summer.
Embrace the weather.
Summer equals gloriously warm weather. Don't stay inside watching a Netflix show you have already seen two times. Get outside and enjoy nature.
Learn a new skill.
Summer is a lot of free time, and sometimes people can get bored. Pick up a new hobby that you can take with you for years to come.
Go out with friends.
The best time to explore your city is in the summer, when shops, bars and restaurants have opened their patios and to provide a spot for you to enjoy the sun with your friends.
GET A JOB!
You are going to want to shop, go to the zoo, go to movies, go to festivals, go to restaurants and go to bars. THAT COSTS MONEY! Pick up a job that you can have over the summer to provide valuable income while you frolic around town!
HAVE FUN.