If you find self-bondage sessions boring without additional entertainment, but do not like the ideas from the watersport section or anything water related is not suitable in your particular situation (eg if you are laying tightly bound on your wife's favourite carpet), you might find this idea interesting.

Just like delayed enema, long butt-plugs cause cramps after some time. The bigger the plugs the sooner and more pronounced the cramps.

Unlike usual "standard" butt-plugs the long one are longer than 12-15 cm (the length of rectum) and much more flexible what allows them to get into the sigmoid. For your information, this is what the rectum looks like.

In my collection, the most effective cramp-makers are:

• 40cm double-sided silicone dildo
  you need lots of patience and lube to get one in
• inflatable butt-plug
  to get the desired effect it must be fully inserted and thoroughly inflated
• 20cm butt-plug
  it's not as effective as the previous two - can be comfortably worn for longer periods (~2 hours)

Additional resources:

• http://www.bartleby.com/107/pages/page1183.html
• http://en.wikipedia.org/wiki/Butt_plug
• DIY butt-plug