In 2011 I spent two months trekking in Nepal, and it was awesome! I can not say enough good things about Nepal. I reached heights of over 5000 meters all without a guide, a sherpa, or altitude sickness. Walking in the worlds highest mountain range is totally possible, regardless of fitness, age, or experience. You need very little to hike in Nepal (see kit list here), in fact you could get all the necessary gear in Thamel, Kathmandu tourist district. Go with a group, a friend or solo; the Himalaya are waiting.
My trek to Everest Base Camp was a very long out and back trip starting and ending in Jiri. I spent the following five days hiking in the foothills of Everest before reaching Namche Bazaar where the altitude really started to kick in. Two days of rest in Dingboche allowed me to acclimatize to the altitude. Feeling strong I made my way to Everest Base Camp and the high point: Kala Pattar, a whopping 5545 meters above sea level.
The Annapurna Circuit has somewhat change with the addition of a road to Muktinah, however it is still possible to trek the whole route from Besi Sahar to Pokhara. The beauty of the Annapurna Circuit is that it is a circle--no doubling back! This trek is not as popular as the trips into the Everest region, however the views are still stunning. The intriguing aspect of this trek is that you climb from around about 800 meters to nearly 5500 meters. Seeing the way the environment changes due to altitude in such a short amount of time is really cool.
The Gosaikunda was my eight day warm-up trek, which included an infamous bus ride. The trek was snowy, cold and awesome!