I want one of these SO BAD! EcoSphere - inside these sealed glass balls live shrimp, algae, and bacteria, all swimming around in filtered seawater. Put it somewhere with some light and they will self-sustain for years, no feeding or cleaning necessary.
The Garden Igloo Kit. in a few different versions. could be a greenhouse +/or alternative to tent accommodations to house a few more summer retreatants. not particularly cost effective though, in my humble view.