I haven't looked at any other tombs yet, but the exposed terrain in the The Great Pyramid is actually at ground level. So there is no real trick here. It is possible to to place ponds and plants in an enclosed room provided that there is no roof over it or another floor. Once you add a floor or roof above it, the game considers it to be indoors and won't place them. So as long as you do the landscaping before enclosing it, there's no problem. - Actually, just did a little more testing and it's only the plants that can't be placed inside, you can still put a pond in with a roof over it!
