"On Holiday also adds two new interesting concepts - postcards and souvenirs. To maintain friendships with those left behind in the neighbourhood, sims can send them a postcard (although I found them hard to find in the resorts I was in) using a special postbox. Likewise, your sims can buy souvenirs of their trip which come back with them. Changes to friendships during the holiday still apply when the sims get home, although (like Hot Date) no time passes while they are away."