For a day trip to Scotland, it's best to rent a car as the public transportation system can be unreliable and may not take you to all the places you want to see. The estimated cost for car rental is around £50-£75 and the total travel time will take around 7 hours.
If you have more time, consider adding a visit to Glasgow, the largest city in Scotland, and the Isle of Skye, a picturesque island known for its stunning landscapes. If you have less time, you can skip a visit to the Kelpies or Loch Lomond to make sure you have enough time to fully explore the other locations. Also, be sure to pack warm clothing and comfortable shoes as Scotland can be chilly and hilly.