Manali would be a better choice if you want to do things. There are many adventure sports you can try in and around Manali like river rafting, paragliding etc.
Hi, You can do both if you have the right package otherwise you can go with Manali. I have attached with this comment few options for you, you can click on them to find more details about them. If you want to talk to someone about the packages simply fill in your details in the form and you will receive a call.
If you are travelling for weekend then shiml coz manali in itself requires lot of time for travelling to reach there..shimla on otherhand is near and can be travelled even in complete one day trip from kalka..Only thing is Manali will be much colder than shimla these days and you may get snow at rohtang but no snow in shimla only scenic views and good food😊😊Happy Travelling!