Going to the beach
There are not many opportunities in the UK to go for a nice stroll along the beach and while the beaches are not the same standard as say LA or Hawaii it is still nice and relaxing to go grab an ice cream dip your feet in the ocean and just chill out for a while.
Go to the Park
I am 24 and I still have an addiction to swings. I cannot pass one without hopping on and getting all giggly. Parks remind me of being a child. Grabbing a nice cool drink and just spending time acting all silly. It is a nice place to spend time with friends and family.
Walk around
I am quite fortunate to live semi near some great big nature parks where you can sit down plonk all your gear, create a nice spot, perhaps do some reading and go on a 20 minute walk. This is something my family does at least once a year (usually in August) generally on a Sunday. We find a spot, then find a trail and get walking. I am not the most nature friendly girl but I genuinely enjoy it.
Sit outside and read a book
I wish we had balconies in the UK. I know they exist but in places like America and Canada they are more common. There is something refreshing and calming about siting on a balcony with a book, some iced tea and doing a spot of people watching while you're at it.
Swimming
Ok so swimming is all year round activity but outdoor pools are perfect in the summer. The satisfying feeling of the cold water against your warm body is hard to deny and it is the perfect way to get some exercise in while having fun too.
However your planning to spend your summer. I hope it is a good one filled with fantastic memories you will cherish forever.