Dino Island

When you reach level 22, you can buy unidentified decoration for 10k money and it is the random one decoration. Since there are a lot of expensive decorations, it is a free profit. Example, buy 40 unid boxes (name of the decoration) for 400k, then random open them and sell those decorations. It generates about 2-5 times you spent on buying, in this case from 800k to 2m, depends on luck. Tried it about 10 times and always made at least double what I've spent.