One thing to remember with macro photography is that flowers move with the wind. So you'll probably want to use a tripod and a lot of patience. It also helps if you can reverse the tube and have the camera below the top of the tripod. You might be tempted to pick some of the flowers to shoot in a light box at home, but beware of doing this as it is illegal to pick quite a large number of wild flowers (maybe most of them). Of course, garden flowers or ones you buy from the florist make great macro subjects - especially the heart of the flower.
One other macro subject that hasn't been mentioned is anything that can be used to create an abstract - kind of a 'guess what this is' photo. Things like cheese graters, wire brushes, jewelry, etc. But you can also get some good macros from common objects like cutlery, food, calculators, bicycle cogs, etc, etc. There isn't really any limit.