Items could include: personalised number plates, makes of cars, car colours, certain letters on number plates etc etc.
Variations on the 'spotting things' themes are: first person to see a semi-trailer has to say "I see a semi-trailer ... semi-trailer I see" quickly before anyone else. Or if you spot a white horse you say "Ping ... I see a white horse" to get a treat. First to see a windmill has to yell 'windmill' before anyone else. First to see a number plate from each Australian state and territory. First to spot a car with one headlight only. First to spot a magpie etc.
Hubbers tip: We played Spotto! Mum and dad would write out a list of things we had to find and we'd yell out "Spotto" when we spotted something on the list! It was funny when we had to spot a lady in a hat ... we had the car windows down and would all scream "SPOTTO!!" and the poor lady would nearly jump out of her clothes!!