The Brain Needs a Reference Point
A lot of what adults end up remembering comes down to what's most personally meaningful. Because kids haven't had enough experiences to determine what's significant, they remember best when they can link one personal experience to another similar one. For example, your child will recall having circle time in the kindergarten classroom if he also had it in preschool. Or he'll remember the outing to the local carnival if he also went to the amusement park.
Make an impression: Bring up the past. For example, when you're on vacation and having fun in the hotel's playground, remind your child about the one at the hotel you stayed at last year. Say, "Wow, this is just like that other one we went to before, but it also has a tube slide. What else is the same? What else looks different?" Bonus: These kinds of chats will help you remember all the fun times too.