Sometimes there is no choice about being busy or not, the choice is made for you - just go with the flow. No matter what your plans were for the day, it is now following a different lead.
We don't know what each day holds for us. God does. If we will listen closely, He will give us everything we need for each day. It's a promise. Matthew 6:33-34 says, "But seek first HIS kingdom and HIS righteousness, and all these things will be given to you as well. Therefore do not worry about tomorrow, for tomorrow will worry about itself. Each day has enough trouble of its own."
It is impossible to pre-plan for every possible situation that could occur (not to mention the wasted energy it uses pondering them) In the end, we must walk out the path laid before us, trusting God and His guidance every step of the way. "Who of you by worrying can add a single hour to his life?" Matthew 6:27
Ultimately, our lives are in His Hands. If He has a specific plan for us (and yes, He does), then won't He have the answers we need to walk it out wisely? He is waiting for us to ask Him to show us. Begin each day by asking Him what HIS plan is for our day, and we will save a lot of time, frustration and disappointment.