Traveling overland can be an amazing and different kind of experience. It is a way to get deeper inside of the culture you are passing through, and it allows for a more localized and closer view of the country. Not to mention it's almost always cheaper.
Southeast Asia is an ideal place to travel overland. If you started at the most southeastern point of Indonesia, you could go all the way to Nepal on boats, buses, and trains, and for the most part, fairly easily.
A popular starting point for many is Singapore, given the country's location as the most southern point on peninsular Southeast Asia. This makes it an easy route north through Malaysia and Thailand to get to countries like Vietnam, Burma, or China.
From Singapore, there are northbound buses leaving all day long. I caught one such bus to Melaka (Malacca), Malaysia, about a 4-5 hour journey. Some might tell you to catch a bus to Johor Bahru, a large city immediately across the causeway from Singapore in Malaysia. It's cheaper to buy a bus ticket from there to anywhere in Malaysia, but in my opinion, it's worth the extra bit of money to take a bus from Singapore.
For example, on a 4.5 hour bus ride from Melaka to Ipoh, a jump-off point for the Cameron Highlands, the driver stopped in the middle of the freeway, grabbed a black duffel bag, and hopped off the bus. I watched in total confusion (and slight horror) as he made his way down the side of the overpass and to the street below, as a man who had been sitting in front of me jumped in the drivers seat and started driving! Nobody else on the bus seemed to think this was weird, so I held my breath and hoped we would end up in Ipoh. Luckily, we did, but I was on the edge of my seat the entire way!
From the Cameron Highlands I hopped a bus bound for Penang, an island off the western coast of Malaysia. I met a couple from Germany on the bus and when the bus dropped us off at the ferry terminal, we boarded the boat together and explored the city of Georgetown. After a few days on Penang, I caught a crazy 8 hour long bus to Krabi, Thailand.
Krabi is another jump-off town, this time to the lovely southern Thai islands. I took a boat from Krabi to Ko Phi Phi, then another boat from there to Phuket. From Phuket, I flew to Bangkok for one night before flying to Nepal. If I'd had more time though, I would have definitely tried to overland it the whole trip! I met way more people, and saw much more of the countries I visited. And I hate planes.