Fun summer with my cousins?

How old are you? Do you have access to a car?

For me, summer time means getting outside as much as possible. You're smart to think up some indoor activities, too, as of course rain can ruin your outdoor plans. I remember doing things like cutting out magazine pictures, sticking them on popsicle sticks and making up puppet shows. There's cookie baking (and eating). How about watercolors?

It also depends on where you live in the world.

A 10 year old in India will be too busy making footballs for the forthcoming World Cup to have leisure time with you, or if he/she is in North Korea, you could always join in with their marching and cheering lessons.
If they are in the USA, they will probably deign to contribute a monosyballic grunt now and then whilst they play their favourite shoot em up game, or if they are Swedish you could all spend a naked day down at the public sauna.
Scottish choldren like to play 'dodge the midge' during summer, whilst English 10 year olds like to spend the day down their local pub, then let a kind policeman bring them home.
Personally, I hope your cousins are Australian, as they could then teach you how to surf in the morning then forage for bush tucker all afternoon before giving up and going to Maccy D's for a drive through supper.