If you want to sandbox the whole environment, Oracle provide pre-built VM images which contain an 11g database, SQL Developer and other tools; more info here, and if you just want PL/SQL the first version listed should have everything you need.
As well as being very quick and easy to set up - no DBA knowledge required - it's completely isolated from any real environments you want to avoid interfering with, can run on any machine and O/S that runs VirtualBox, and best of all, if (or when) you really mess something up you can trash it and start again.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…