I did the base camp trek in 2003. Excellent trip. Well worth the experience. Check with Last Frontiers out of Kathmandu.
http://www.lastfrontierstrekking.com Ask for Thubten Yeshi as a guide. He grew up in the Everest region, studied as a monk, great English, superb guide. Probably a good time to go because the govt. just recently reached a peace agreement with the Maoists rebels ending several years of violence.
Just did Machu Picchu in August. Try SAS Travel out of Lima, reputable and helpful.
Have also done Kilimanjaro (Africa) and a number of treks in the Alps. All are great places. Consider adding on a safari if you do Kilimanjaro.