Simples, you need to build a walkway at least 25 blocks away from spawn, then just build like a 12x5x3 building (add a roof too so on the outside it's really 14x7x4) and mobs will spawn day and night
WARNING: Add torches to the top (you should know how get torches and if not...
Warning: Spoiler! Charcoal
) or else mobs will also spawn there!
I'm currently making an extension so they spawn above the area and then fall down to their death on the primative ones rooftop, tho my trees are taking forever to grow (luckily I get about 50 skeles a day so soon I shall start spamming and then it'll be time to ignore the cobble and make a nice wooden land
)
I'll post screenies of my own tomorrow and you'll see what I mean
Even better... If it doesn't kill my cpu to run both fraps and minecraft at the same time... I'll do a vid tour or something! (It'll take all of 2 mins lol)
EDIT:
minetrade.danomagnum.com/files/392/
I took to editing the save file. There are 3 new islands around which enable you to get to the nether (nether not yet edited)
They require a large amount of resources to get to so if you do plan on trying it out you'll be using your cobblestone generator a lot
New Tasks for the 4 islands:
Sprint for 10 seconds
Grow a large tree and set it on fire
Make a piston
Get a slimeball (you'll have to figure that one out but it IS possible)
Make a sticky piston
Create a SSAGE (Static Sand And Gravel Extractor)
Make a portal to the nether
Escape to the mainland using the nether
After escaping to mainland, make a bridge back to your island (the more fancy, the better)
Populate your island with passive mobs
P.S. You'll find two islands easily, the third... good luck!