When Claire was that age, she was quite content to stand up in the trolley. She wouldnt have a bar of the seat thing, but was happy to stand in the main bit of the trolley, or sit in there with a book or one of her toys.

Now, she isnt as keen on it, so she walks next to me but I also ask her to find me things on the shelves. Just stuff that we are standing right in front of anyway. It keeos her busy, although she does end up getting a bunch of extra stuff that we have to put back onto the shelves.