An Instructional Tease: Advanced Anal: Household toys pt.1
Well, a hammer has quite a large flared "base" when you insert it handle first, while a shampoo bottle, phallic as it might be shaped, doesn't have any base. If it was shaped, and you could tie a string around it so you have something to grab if it slides too far in, and if you are very sure that the string wouldn't come off and that the top wouldn't pop off either, then it could be safe. A hammer is obviously safer (as long as its clean!).
Cucumbers are the most phallic vegetable out there, but they aren't the safest to insert in your ass. Even if you never let go of it so it couldn't slide all the way in, a cucumber could break, and then you'd be left with part of it in your ass. Grapes, on the other hand, are small enough to force out without needing your hands, no matter how many you stick in!
The advantage of a vegetable over, say something plastic, is that, if it got stuck, eventually it will start to decompose, making it smaller and softer, so you could eventually pass it easily. This could take a bit of time, though, so if you understand and accept the risk, you could go ahead and try a cucumber or carrot.