Some of these ideas are Kenya-specific, but I'm sure you'll find similar things to do abroad.
1) Visit the David Sheldrick Wildlife Trust: Plan to visit around the time all the baby elephants are out to play. If you have extra time, make sure to visit the giraffe centre too, and then head to one of the many fabulous restaurants in Karen, like Talisman!
2) Take a walk: Again, if you're in Nairobi, get up early on a weekend and head to Karura forest. (I'm ashamed to admit I haven't been yet, but I'm planning to soon!) I've heard it's absolutely beautiful!
3) Sign up for a class together: Be it zumba or cooking or shooting or bartending or painting or Irish, learning something new is always fun if you have a friend with you! If you've always wanted to learn something, but felt silly at the thought of signing up, rest assured that it'll be easy with a friend.
4) Wine and canvas: You get to drink AND create something awesome together. How cool is that? Most cities have their versions of this I believe, and it's quickly catching on. I was excited to learn that Nairobi does this too!
5) Paintballing: Enough said.
6) Head to a restaurant that does karaoke: Personally, I don't think I'd go for this for fear of all the glass around me breaking, but I've been there while friends have gotten up and had a blast, and it was lots of fun- for all of us!
7) Plan a book club: You may know where I'm going with this, but trust me- you don't. If you've never gotten together a bunch of your closest guys and girls, handed the most serious person in the room a ridiculous romance novel and asked them to read out loud, you're missing out! Of course, book clubs can also go in the normal direction if you'd like...
8) Road trip: If you have enough time, plan a road trip and stay in another city for two or three nights! It'd be a fun getaway for all of you. My personal recommendation? NOLA. If you're within driving distance, of course.
What are some things you like to get up to when you're with your friends? I'd love to hear from you!