Find a “magic” stick and have your 3-4 year old walk around the neighbourhood with you finding letters and numbers that you see (eg. “Can you find the letter “X” on that blue car?”).
Great on rainy days too – suit them up in their rain coat and rubber boots and off you go. Letters/Numbers can be found everywhere, such as on store signs, mail boxes, sign posts, license plates, and vehicle advertisements (watch out for car alarms and for potentially annoyed people staring at you while your kid points a stick at their license plates).