5 x 1kg bags of rice.

Go back and buy another 5 x 1kg bags of rice.

Do it again.

And finally, with the last of your money, buy 4 more.

Its a large enough object and repetitive enough that they will actually notice and remember it. Splitting it into multiple transactions makes it seem more unusual.

Aldi sell 1kg bags of plain white rice for 52p each, so you can buy 19 bags and have 12p left over.

That much rice is almost 70 million calories - 70k kcal