This is an area that used to be very dangerous and most would not have considered a viable alternative to the Rumichaca border crossing near Ipiales. This appears to be changing and the area may be opening up for travelers. There is still some instability, but as of 2014 it seems relatively calm. Check the current situation before you go. Don't expect to see much jungle, however. Oil companies have moved in and appear to be rapidly laying waste to what little forest is left.
You can cross the bridge into Ecuador on foot. The actual border crossing is 3 km into Ecuador (both Colombian and Ecuadorian parts). There are colectivos from the bridge, or you can walk (not recommended on account of the excruciating heat/humidity).
The San Miguel river, briefly, as you cross the border.
Not much of anything.