We get this question a lot here and lots of people recommend the Ninja 250 or Honda Rebel 250. A 250 is small and light and easy to manage, especially important for beginners. But a 250 can't cruise at highway speeds. I think a 500 or even a 650 would be better. It's bigger/heavier but not that much, but you can go faster and farther. You could do a long Sunday ride or a weekend camping trip.
There are lots of motorcycles in this size range. You want a 'standard', i.e. not a dedicated sportbike. Kawaski Ninja 500 or 650. Honda 599 aka CB 600 aka 'Hornet'. Suzuki GS 500 or SV 650. Actually there are a lot of models in this category. Some are even designed as trainers--the Buell Blast 500 (no longer made but used ones are around), the BMW F650, etc.
You want a used bike as your first bike. I think a person should ride for at least six months or a year before getting a new bike. After that time, you can sell a used bike for about what you paid for it, and by then you'll have a much better idea of what you really want.