I'm planning on having a motorbike with me when I hit the road, been riding for almost 50 years and I'm not ready to give up yet. A lot depends on what you need it for. Like someone earlier mentioned the Honda trail 90. It's a good choice and even though they're not made anymore there are still a lot of them around. Unfortunately they have become "collectible" in some people's eyes, and nice ones sometimes go for more than they cost new. If you're going to pay that much it should be a complete rebuild/restoration. I believe some of the Chinese companies are making knock-offs now, probably for less than a nice older Honda. One drawback with that bike is the top speed. With the right gearing they might get up to 50 MPH, but that's pushing it.
I have three bikes myself now, eventually I'll have to cut back to one. My requirements are something for light off-road trail riding, back country roads, but also something that will go 55 or 60 if need be. It also has to be easy for me to load by myself. I've ruled out one of my bikes that's too big for that, so now I have to decide between a Kawasaki 250cc single street bike that' I've modified for light off roading, and a Honda 350cc twin that was originally made for light off road use. Either way I'm looking at getting a hitch mounted motorcycle hauler that's rated for at least 500 lbs.
Hope that helps.