I did some experimenting. Definitely the fishing for Mithril-Bound Trunks are the best option for F2P accounts. Unfortunately you cannot dedicate one character to fishing because Warband Banks are not available for F2P. Fortunately, I was pulling up Mithril Trunks even with a fishing skill of only 35. The beaches of Blasted Lands & Tanaris were the easiest for Sky Riding. Stonescale Eel pools were also excellent for fishing Mithril Trunks. Snapper, Blackmouth, & Greater Sagefish only gave Iron Trunks. I even tried skinning level 15 wolves with a level 11 character and at 110 skinning (because I was level 11) the skinning was considered grey for skill up chances. So I can confirm it seems like it's solely based on how far your skill is below your level (times 10). That being said... If you're really wanting to hit skinning 300as F2P, only learn classic skinning and do NOT skin anything but classic beasts. Once a month Darkmoon Faire will offer the skinning quest for a +2 to skinning, but it will apply to whichever most recent expansion you have trained in... That's a long wait, but it is possible. I can confirm that even being trained only in classic skinning, I skinned a Darkmoon Faire wolf and it immediately gave me trained in Cataclysm skinning with a skill up.