Things to do alone:

(re)decorate and organize your room

book, plants, and photo image Inspiring Image on We Heart It
start by getting inspired (I love watching room tour videos) and getting rid of everything you don't need anymore. If you want to get creative, you can make diy room decor.

start a bullet journal, art journal or travel journal

Have fun with this! You can get very creative and doodle or paint in it, or you can just write about your day.

try new recipes

food, healthy, and salad image food, chocolate, and cake image
if there's a special occasion, you could try making a beautiful cake

go for a walk

try a new makeup look or hairstyle

have a spa day

aesthetic, bath, and candles image self care and bullet journal image
do a hair mask, face mask, moisturize your whole body, paint your nails...

sunbathe to get a nice tan

chilling, girl, and summer image Mature image
make sure to use sunscreen!!

do at-home workouts (there are so many youtube videos on this), don't forget to stretch and drink lots of water!

practice drawing or painting

prepare yourself for the new school year

study, motivation, and school image book, books, and coffee image
you can quickly revise your subjects and make sure you have all the school supplies you need

Things to do with friends

have a photoshoot!

girl, friends, and summer image friendship image
it can be very fun to try to recreate celebrity photos

have a sleepover or a movie night

netflix, pink, and wine image people image

have a picnic

food and sweet image food, aesthetic, and picnic image
it's so much fun when everyone brings something to eat/drink, so you have lots of yummy food

go out, if you're old enough :)

aesthetic, theme, and dark image girl, party, and friends image

watch the sunset

do karaoke at home

bake something (and eat it😋)

Image by 𝚊𝚙𝚛𝚒𝚌𝚘𝚝𝚊 food, Cookies, and chocolate image

do eachothers makeup and hair

go shopping

babes, iloveyou, and Peluches image amour, babe, and best friends image

visit a city near you and do all the tourist attractions (like museums, old buildings...)

go swimming

have a bonfire

friends, girl, and fire image fire, marshmallow, and autumn image

those are all my ideas, I hope this inspired you and I hope you have a wonderful summer!
You can check out these collections for more inspiration