nice! i did the inca trail hike to machu picchu. the hike itself will definitely be difficult but totally worth it. do you know what company is doing the tour? if it's SAS see if justino or silverio is your guide and say hi for me. he's a great guy and will totally take care of you. i didn't do the rafting or horse riding (a fried did this). lake titicaca is absolutely awesome, but there isn't much going on in puno (the town you'll be in). you'll see the floating islands, and hopefully get out to amantani and taquile islands. if you get out that far you'll see bolivia in the distance
the altitude will mess you up at some point. find 'soroche' at the local farmacia in cusco.
make sure to not drink the water (like mexico). ice is never offered so you'll have to ask for it.
here's the album.
Picasa Web Albums - Stacy - PERU#