We went there expecting some amount of trekking, waterfall and wildlife. The plains are not really plains per say, they are quite undulated and the path that you walk on goes through forests, plains, and literally rocky roads. However the "road" comes to an amazing halt at the end only to witness a magnificent / breathtaking view of what is called the worlds end. It is recommended to get there early - we started from Nuwara Eliya at 5:40 and got to the gate of the Horton Plains at 7:00. However, I wish we had gotten there sooner, the place opens up at 6:00 AM. We did get there in time (before the mist / clouds covered the view) but getting more time at the world's end would have been better. Also, the walk back would not have been as hot, if we had left sooner. Also there are quite a few pits stops that you can make - mini world's end, Baker's fall -- which is absolutely amazing -- we took the steep path to the bottom of the waterfall which was totally worth it.
Note: The early mornings are cold / really cold so it is recommended to wear warm cloths and carry a hat for sun protection. Your walk back might be hot and sunny, so be sure to have sun screen on as well. There is only one place to eat at the start of the trek so considering your trip will be at least for 4-5 hours, its recommended to have your hotel give you some take away b'fast. Also no restrooms along the way so be prepared for that as well.
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