Rainbow mountain is definitely place you have to visit when you are in Peru. The easiest way is to take tour from Cusco, it cost 80 soles and includes bus both ways, breakfast, lunch, entrance ticket and guide. Don’t buy it online! Prices are twice more expensive than when you buy it in the city center. From drop off place to the top is around 3 km but it’s the hardest hike I’ve ever done in my life. The highest point you will be is around 4900m over sea level so better don’t plan it as a first thing in your trip, give yourself time to acclimate.
It’s good to leave Cusco around 4 am, I left around 6 am and I was walking with tons of tourists by my side. It’s not bad actually until you want to make a picture.
I’ve done this trip during winter, in July so it was quite cold on the top but on the way was extremely hot as we had sunny weather with no wind. So if you want to do Rainbow Mountain the best time is June, July, August. Rest of the year can be more cloudy and rainy.