Absolutely an online savings account. Most brick and mortar banks offer pathetic interest rates. If you put it into HSBC or ING you can get around 5% interest. It generally takes 2-3 business days to transfer the money to your checking account.
I just checked and HSBC Direct is offering 6% APY interest rate on all new deposits, valid through April, after that it goes back to the regular rate of about 5%.