You put a lot of restrictions on what people could answer with. People have suggested thieving, hunter, and agility and you've either rejected or ignored them. You say you don't want to waste it on a skill where getting higher levels won't unlock anything useful, yet you also say you think prayer would be a good option. You've made a lot of restrictions, rejected every suggestion, contradicted yourself, and have made clear your only goals right now are 100 dg, 99 craft, 99 smithing, and having fun. If you want to have fun then do whatever you want with the experience. Go with whatever random gut feeling you get. If you want advice on the best option for maxing out, go with agility since you have the charms for summoning already. If you want the best out of your current goals, crafting is the most expensive and is marginally faster than smithing.