If just being sent somewhere to get a glimpse (not being able to interact or change anything), three different eras in time comes to mind.
1. Back in time to the Dinosaurs era (Mesozoic Era/The Age of Dinosaurs) and to watch the Triassic–Jurassic extinction event when they died.
2. Back in time to watch Jesus rise and fall (was i allowed to bring a camera or was i only allowed to observe like a spirit?)
2b. If i would be able to at least leave a note or say something (but still be invisible), i'd go back to 1939 and tell Hitler to change strategy a bit.
Maybe show Stalin some photos and videos of his future (2000-2020) and persuade him to ally with Hitler.
3. Forward in time, to see how mankind is faring in either 2100 or 2200.
4. Jeanette.