Well, interesting question...I think everyone has had a little of this dream....anyway, here is my list (it would take more than one month):
1) Destroy all nukes, except 20 for the US n 20 for Russia.
2) Destroy all drug plantations, replanting them with food harvests.
3) Remove all tyrants from power.
4) Destroy 97% of the world´s weapons.
5) Clean up most of the pollution. Compress all radioactive waste, litter and junk dumps into huge balls and throw them at the sun. At superspeed, clean oceans, rivers and lakes.
6) Transport shiploads of medicines for poor countries.
7) Transport shiploads of food for poor areas.
Open new rivers across deserts.
9) Build huge dampers in Indian Ocean coasts to offset tsunamis.
10) Throw tons of birth control pills into the lakes n rivers of areas with too much population growth.
11) Build Huge city-sized buildings, like Krypton´s, so people could move into them and cities would not spread horizontally, keeping environmental disruption minimal.
12) Demolish most of current cities (after empty) and free their land, turning them into harvested areas.
13) Build huge solar panel satellites to provide us with clean energy.
14) Work with Interpol, find out the main criminals and crooked politicians and jail them.
15) Get a holiday!