The opposite is outdoors.
Indoor sports are just indoor sports. But if you want to get specific then you might say indoor soccer, indoor tennis, indoor football, etc.
The word 'indoor' is generally used as an adjective, as in 'indoor activities', indoor swimming pool', etc.
Indoor soccer
In college you can play: Indoor Soccer Indoor Handball Indoor Swimming Indoor Floor Hockey Ice Hockey Basketball Football Baseball Golf Indoor Tennis Indoor Volleyball Dodgeball (Intramural) Dance Team Cross-Country
an indoor stadium
You can get information on indoor gardening at Indoor-gardening-guide.com. You can also visit Gardenhobbies.com.
National Indoor Arena is an indoor arena in United Kingdom and was opened on 1991.
Indoor temperature is the temperature indoors.
The Indoor League was created in 1973.
Indoor Sports happened in 1987.
Indoor Sports was created in 1987.
Indoor Living was created in 1997.