If you do want to visit Skye then book accommodation now, it's very busy as are the roads.
Mull is less so but with the recent ferry breakdown crossing from Oban needs to be booked in advance. You can always cross from Kilchoan or the Corran and Fishnish without booking. Tobermory is well worth a visit and you can also do a boat trip to Staffa, basking shark spotting and there's a good chance of spotting a golden eagle if that interests you.
