1) Go into File/Build Game, then click on the Level Settings tab, then click on Global Script, you should see a sky change option (click preview before selecting it). Then click OK, make a small level, and test it.
2) Either make a wall around it (Walls Large and Small are in Segments/WW2 or Sci-Fi/Scenery. Or you could use the search bar and search "invisible wall" (without the quotes).
3) Search in the Models and Media pack for "plants" (without the quotes). Or you could take a look at
http://www.thegamecreators.com and look around in the side bars (you can hover your mouse over them to get lists of products in that category) until you find TreeMagik.
I hope this helped you, and welcome to what's left of the forums; you'll be seeing a lot people leaving during your time here, but a handful of people (including me) will stay.
~AI
AmericanInfantry: Splinter Cell worshipper