I didn't realise the walk into Machu Pichu was restricted / guided only. Can you not do it independently?
Not any more... Cuzco's odd, it's the only really 'touristy' place in Peru, but somehow manages to transcend all that and still be astonishing. The Inca Trail is lovely, if only for the view from high up of the ruins at dawn. If you are going to do some trekking, give yourself a decent amount of time to acclimatise first.
If you fancy something a bit wilder and have some trekking experience, take a look at the Ausangate Circuit trek which is around a day's journey by bus and truck from Cuzco. It circumnavigates a whopping great mountain and climbs over three passes of around 5,000 metres. It's quite stunning and, when I did it around ten years ago, utterly deserted bar the odd llama and a few scattered settlements/fincas in the valleys.
As far as elsewhere goes, I think Bolivia is astonishing. It's quite easy to get down to Lake Titicaca and on to La Paz from Cuzco and I'd definitely look at doing that. And the mountaineering there is good too if that interests you. Colombia's a mad place, though not necessarily in the sense that people tend to think, lovely people, very friendly, quite beautiful.
But there's so much to see just in Peru. One thing, if you don't speak Spanish already, it's worth learning some before you go. It'll make your life much easier in South America as not many people speak English and, out in the sticks, the usual first language is either Quechua or Aymara, so even the Spanish isn't always that great.
Have fun.