Processing time for Scotland visa and 10-day itinerary recommendations

Hi, flight ticket booked for Scotland—10 days in May! [:)]

First question: I see on websites that starting from 01/04, a visa might be required to visit Scotland... How long does it take to get one, and what’s the cost?

Second question: What are the must-see spots, and what itinerary would you recommend for 10 nights?

Thanks for your tips! [;)]