One thing I found interesting in Skyrim was the Skooma Dens across the world, notably the one in Bravil. I remember going through the entire game collecting bottles of Skooma and when I finished the game, I sold them all and made little over 12,000 Septim. Not only are they good money makers, but it enables you to roleplay a skooma addict, who would search for the gold for his next fix, doing everything he is told, or commiting horrendous crimes to pay for his addiction. It's also kinda interesting when you are sneaking around a castle in the cities and find the counts/countesses personal Skooma vault. Can't remember which city now, but a hidden door in one of the castles lead to a wine cellar with about 10 bottles of skooma xDD
I hope Skooma makes a big return in Skyrim.