How to unlock: 1. Follow Pepsi 2. Checkin at 2 times at any venue with a summer fun main category such as: "Beach", "Park", "Baseball Stadium", "Lake", "Water Park", "Theme Park", "Surf Spot", "Zoo or Aquarium", "Music Venue", "Scenic Lookout", "Hiking Trail", etc OR 3. Checkin at 2 times at their recommended venues 4. Previous checkins to summer fun venues count
Any "summer fun" venues or recommended venues will count towards this badge
Unlock Message: "Beach, BBQs, Parks and sun - what could be better? By unlocking this badge, you're eligible to win awesome prizes. Now time to relax and unwind - it's summertime!"
In fact the description on their foursquare page says "Summer Time is Pepsi Time! Follow us on foursquare and check in at summer venues to unlock our Pepsi Summer Fun badge. Keep an eye on the page for more summer fun coming soon!"
Then on the badge when you unlock it, it mentions "beach, BBQ, parks and sun." that leads me to believe if you check in to a beach or park (which is where i unlocked mine) or other summer type venues, you'll get it.
I did some testing at local syracuse venues. The following venue types unlocked it:
1. Park 2. Beach 3. Baseball stadium 4. Lake 5. Water park
I'm sure there are more but this is definitely worldwide and not limited to their recommended venues. Look at their recommended venues for ideas on venue types that will unlock it.
I unlocked it in Campo Grande, Brazil after checking in a venue categorized as Park. It's clear you only have to follow Pepsi and check in to such places to get this badge.