I can't bring myself to go up to that counter and rent it

If I, an adult male, can buy Zack & Wiki, you can rent Viva Pinata.  Do it.

you know it may look like a kids game but to be good at it you have to be a Man or a women....Fully grown to grasp every concept and complete the harder challenges 

You could just buy it online, because Viva Pinata is awesome.

There's nothing to be ashamed of... Plus, you could always act like you were getting it for a friend, nephew, cousin, or a sibling.

This is going to sound cliched as hell, but don't worry about what other people think.

Well, there are worse things to be embarrased of...like going to a movie store to rent movies for your little sis...jrenting the Power Rangers movie is embarrasing enough.

I own it and I don't regret buying it one bit. 

Why not by something else at the same time to spread it out? I'm sure a dirty magazine and some baby lotion would help.

This is the same feeling I get when, I buy pokemon gameboy games.
Actually I stopped buying them about a year ago, just for that reason. lol

Don't be ashamed, no one gives a shit. Besides, people that have played it know that a kid could never master it.