I have done a MICU and PICU rotation--neither was a Sub-I, but I am planning to go into critical care so I took the experiences pretty seriously.
I'll mention some things that helped me...some are specific to the ICU but a lot of this is true of any medicine rotation.
In regards to procedures, it really depends where you are. In an academic center, doing procedures is more difficult because there are just so many more people who want/need them--residents, fellows, mid-level providers, etc. In the community hospital where I rotated, the residents had gained more experience earlier in the year because they didn't have so much competition and they were happy to let me take procedures. Overall, the best way to get to do procedures is to always be around so you know they are considering a procedure and also so that the team will be more likely to ask you if you're interested. If you don't get to do one, stay around and watch anyway, so they can teach you and then possibly let you do the next. Your overnight call is a great time to do procedures, so don't sleep unless the residents do, and if they do, make sure they know to wake you if an opportunity arises. If respiratory therapists typically do ABG's in your hospital, get to know them and tell them you'd like to do as many as you can.
Another reason to get to know a resp therapist is that they know a lot about the vents. In my experience they are very willing to teach. And of course, make friends with the nurses. The nurses in the ICU are typically the best of the best and, even more than on a regular floor, they are invaluable. You really want them to trust you so you will be the first person they notify if something happens with one of your patients. This will allow you to tell the team of the problem and your own ideas for solving it, which is a great opportunity to impress them.
One nice thing about the ICU is that all your patients will be in close proximity to one another, so you can keep up with everything that happens through the day (and things change quickly). Try to talk with every consulting team directly as they are deciding on their recommendations. The residents will really appreciate that you actually know the reasoning behind the chicken-scratch note in the chart.
There is a lot of variety in the ICU. A lot of your patients will have respiratory failure, so know its main causes and how to deal with the vents as cadoc mentioned. In fact, I would know a few basic things about the vents before you even start, or you'll be lost on rounds. But almost all of the residents I worked with had trouble with vents, too, so they are happy to review it with you and don't expect you to be an expert.
The other big thing is shock/sepsis. Know the causes of shock, the various types of pressors and when they are indicated, and management of sepsis (lot of controversy here).
Review how to read chest x-rays if you're rusty. You'll check them every day.
Hope this helps. I was expecting to be overwhlemed in the ICU but didn't find that to be the case at all. I hope you have the same experience!
What should I expect in my MICU rotation? Will I get to do lots of procedures? Will I get pimped to death on PCWP values of the Swan-Ganz!!! What should I expect as a Sub-I. Thanks 
