Really fun to do Nasca, Dave and Merry Jo are right.
To be honest, with the Lonely Planet you have all the infos to do all of it by your self.
Great idea to the Inca trail, it's 4 days...never did it...maybe one day. I'll send you a PM with my friend in Lima.
With limited time, what you want to do is fly to Cusco, tickets on lan.com, it's the safest airline. There is a full range of hotels in Cusco. Do the inca trail and come back by train. Leave two full days for Cusco, it's a amazing place.
For Nasca, you can combine this with the dunes of Huacachina, and Ica for some great piscos. Got all the adresses to do it properly, this takes 3 days...Nasca, Huacachina, Ica and Paracas. Busses are safe, cheap and nice to do this part.
You may want to add some Amazonia...I'm not a big fan of this, but that's everybody's call. Also worth it is Puno, easy to reach either by bus or plane.
Why don't you try to make a trip combining Peru and Bolivia...I was stuned by Bolivia this summer.
For the diving...ok...forget most of caribean it's hurricane season in October/November...I would not risk it. You need to be below the hurricane belt. You could try Los Roques in Venezuela, went there in July, it's amazing. Also there is Cartagena or San Andres in Colombia (never been), or try some diving in Brazil...but than it is becoming more pricy.
We are off to Vietnam this week-end, and monday I'm in Hong Kong and than Africa.