Off the main lobby, where you see the huge windows,
is the Arusha Savanna. There is a observation area out there. Sometimes they have a guide there with the night vision goggles sometimes not. Generally you see lots of animals out there, however it all depends on timing of when they fed the animals last. If they recently put food out you are more likely to see the animals.
Since your at Boma, you can walk out past the pool area, to the very end you will see more savannah area. Sometimes you can see the bongo and other animals there. It all depends when they put out food for the animals.
On the second floor (which is the lobby floor) they have observation posts for the arusha savana. They have a binocular mounted so you can use it to see the animals. Its not the same as the night vision goggles, but will magnify what you can see. As you are walking to the observation area, there are windows you can look out and possibly see the animals better.
Have a great dinner.
Connie