Any suggestions on where I could put it other than under the mattress. Bank account will not work, she will know the money is there and spend it, etc.
ING Direct is a good internet account. Paperless statements. Online notification through your email. If she reads yours set up one through Yahoo she will never find out about.

About telling the wife, don't, just name her as the beneficiary and then if you do get hit by a bus she will get the money to bury you.
Open up a money market mutual fund in your name only. Then she cannot touch it. She can see it if she opens a statement, but she cannot get to it, so all she will be able to do is to gnash her teeth and snarl.

You may be able to opt out of receiving paper statements, so then she would know nothing about it at all because she would not know where to look.
Opening any savings account in your name only will suffice. I prefer www.ally.com myself. They pay decent interest.

I would NOT opt for no paper statements. You must let her know about the account. If you get hit by a bus, you want her to know where the money is.
Buy a 5 year cd.
Money market account is the best in your name only.