How awesome for you! Congratulations! PICU is an amazing place to be for learning. I hope that the unit gets you involved and loving PICU.
As for advice, I would say start off with what you think a good peds nurse would be. Review developmental practices and just focus on being comfortable with kids. A lot of the more technical stuff you'll get in any job (central lines, meds, drips, etc.). Like any preceptorship/practicum, of course act interested. Nurses will pick up on your level of interest. Be careful of what you say, as you'll be judged on what you say and how you act quickly since you don't already have longstanding relationships with the nurses.
This is silly, but I was with a nursing student last month who spent most of the time on the cell phone or the computer working on assignments. Practicum can have slow times (duh!), but if you've got a personable preceptor, take advantage of it! Ask questions, be interested, volunteer to get your hands dirty.
I hope you enjoy PICU nursing :)