If you can only visit one place in Prague, it would have to be the Charles Bridge over the Vltava River. The river pretty much divides Prague into two; the east side is the Old Town and the New Town; the west side is the Castle and the Lesser Town. And it is from the bridge where you can see each part of the city from a distance.
They say the bridge is very busy during the day time; we didn’t really find out because we were never there in the middle of the day. We were there either early in the morning, or early evening, when there was only a few (or none) of temporary shopping stalls set up. I suppose it would be a nice experience when it is super busy, but it would take a lot longer to get through the bridge. Besides, we would much rather to see the cityscape at night with the lights on; it’s much prettier that way. It is also quite neat to be there when there is almost no one around, i.e. before 7am. That is the time when I could have the tripod anywhere on the bridge, and take long-exposure pictures. The bridge is quite safe at night. They have a sign saying that it is 24-hr monitored. I spotted the two plain-clothes guys patrolling early in the morning; it was easy to tell because most other couples there that late at night were holding hands.
Another place where we enjoyed the river was the sidewalk next to the river on the east side. There weren’t as much light posts installed, so probably not a very good idea to be there after dark. We were there on our last day around sunset time, enjoying a little bit of sunlight that had been terribly missed.