when Solomon said its all vanity he must have done everything he could possibly do in this world then. since he sinned against God i can imagine all the things he did were not necessarily Godly. But what i am sure is that some were in fact things we do today whether Godly or otherwise but all in the name of having fun. Finally with his great wisdom he then concluded everything is vanity including his own wisdom! So what did he decide is important, fear God and keep his commandments;for this is whole duty of man. For God will bring every deed to judgement, with every secret thing whether good or evil.
so sometimes i look at my life and Solomon speaks to me about life. I decide that i should forget chasing after the wind. To toil and not have pleasure is vanity. To withhold something from a neighbor is vanity. he says whatever we have, like we all know, we will leave it here on earth. We in fact don't need anything acquired here in our next life including wisdom, knowledge,wealth etc. He also tends to doubt our importance as we die like animals, so we should not feel so important after all. so i say again whoever ate my cheese please come for more, life is short. we should live it fully without withholding anything as there is no carrying forward part of our life.
NB, just fear God and obey his commandments :)