10,000 yen a day is a reasonable amount to have if you are planning on sightseeing, eating out, drinking etc(6000 yen without booze). If you get your lunch at a convenience store then that is only about 500 yen, then splash out in the evening (about 2-3000 yen). There is loads to see and do in Osaka so if you don`t spend it all on one day, roll it over till the next day. You won`t find many places that accept cards, Japan is a bit behind like that, strictly cash, just to be on the safe side. Also, travellers checks are not much use here either. As for presents, there are loads of places selling souveniers pretty cheaply(500-1000 yen). It depends what kind of things you want to buy as presents.
If you run out of money then you can use foreign credit cards in the ATMs in post offices and citibank (if there is one). Enjoy yourself.
Source(s):
Live in Tokyo but have been to Osaka a few times