There is something on this site for all children K-12. It was not designed for just gifted students, but would easily work for a gifted classroom.
Students can participate individually or as an entire class.
The directions and process are easy to understand and can be quickly accessed. Most of the fieldtrips are science curriculum based and I visited the Oceans site. There were twenty-one stops on our fieldtrip. It had terms to learn as well as many different concepts. The teacher or student may also provide feedback.