If they are your friends, using 'what are you up to tomorrow' is a friendly way to ask them! They also have the chance to say they are busy tomorrow without causing you distress ( they could down a direct invitation). It can also help your friend feel good--you want to know what is going on in their life! 'What are you doing tomorrow' is more direct, so if you feel like being direct (like asking them to a movie) you can use this instead. But, I think the first option is much more friendly.
Hope it helps :)