For a young child, trawl charity shops and get a big old lady's style handbag. Fill it with other charity shop finds like a glasses case with pair glasses in, an old 35 mm camera (found one for 50p), a purse, notebook, pens, old make up jars and bottles, pack of tissues, fold up hair brush, compact mirror, bunch of keys etc etc etc
All the junk you would normally find in a handbag plus some extra little surprises such as small picture book, pocket sized photo album with pictures of your friends and family, mini packs raisins, small tins and boxes with little surprises in.
Load it all up then casually give it to child in the back seat. "Oh just hold my hand bag for me would you?"
Watch surreptitiously in rear view mirror as they unpack and mess about with it all for about 2 hours. Ours cost about £10 to put together and is one of the best 'toys' I have ever got for ds. Only bring it out on long car journeys, or the magic is lost!