Silver Keys: Go to the Rose Cottage in the Hobble Cave quest. There you have to use your spade to dig up this key. Dig near the flowers. Get more gold by cutting their thorns. Note that you can do it as many times as you wish, just don't complete this quest and get unlimited Silver Keys!
Fire Heart: When you try to solve this puzzle, try to turn all stones to moons, instead of suns. It's much easier!
Weapon: To get a very powerful weapon early, do the following. In Oakvale, talk to the Chicken Competition guy and to the pirate, Then you should dig where he says and give money to his wife. Now you can dig, but you will be doing this just to get the axe and gold!
To get easy money, play the Spot the Addition game in Twinblade's Camp. You should make saves very often. Practise with low amount of gold, then you can increase it when you improve your skills, of course.
Get XP points! Kill the group of white summoners at the snow village. Then run very fast to Archon's Shrine. In this case, you don't use your combat multiplier, as you can notice. Now go back and repeat this trick. You'd also better not to get hit, use Multi Arrow and bow for this purpose.
Get a powerful sword! If you solved the Fire Heart Puzzle, you can get to the Lost Bay. There you will find a demon door and you will have to give all your keys to enter it. If you have less than 30 keys, don't do it. But if you have, then you can go there and find a chest with the sword which is much more powerful than Aeons or Avo's Tear.