World Association of Zoos and Aquaria (WAZA)
How the Lion should be kept »

Legal requirements for the keeping of large cats vary greatly from country to country.
Even if lower national standards apply, it is recommended that an outdoor enclosure of 500 m² per pair, or a female with offspring if kept separate, should be provided. The enclosure should be enlarged by 50 m² for each additional adult. Covered enclosures should be at least 3.5 m high. In cold and temperate climatic zones, an indoor enclosure of at least 15 m² per adult animal is necessary, where a minimum temperature of 15°C should be maintained.
Outdoor enclosures should be well drained. They should have natural soil with a grass cover or a layer of sand or marly limestone. Enclosures must not be empty but suitably furnished to meet the lions needs such as the following: Visual barriers to enable the lions to hide from one another, e.g. rises/dips in the ground, logs, rocks, plantings, but these should not allow one cat to corner another, there should always be an escape route. Trees for scratching and shade, but not too near the fence.
Platforms or a hill for sitting on and overlooking a view - if possible, one in the shade and one in the sun. Plants that smell strong and/or attract insects add interest. There must be several resting places providing some protection from inclement weather.
Lions are social animals and should be kept in groups of a male and several females, or in bachelor groups. In smaller enclosures females should be separated before giving birth and initially when rearing. There should be a cubbing den; dry, dark, quiet, warm, draught-free, positioned in the least disturbed part of the facility. A wooden den in this for giving birth is an option but not essential if the den is well designed.

Grass, trees, hill for sitting...