Btw, I'm all for doing anything that includes taking the lens cap off in advance. When I first tried it out, I got halfway through my first roll, taking fabulous pictures, before I realized that the lens cap was on the whole time. Duh.
Whenever I've gotten a new Holga, that's the first thing I do: remove the lens cap. It doesn't do a thing, and an scratches and dust will just add to the whole lo-fi things anyway! It's always a problem since you are viewing through a different lens than you are taking through.
You can though use it in place of the card that choppert mentioned.
I use the shutter release, and have super glued a small nut in place to screw it into. you can find one fairy easily that will fit the threads on any shutter release... not sure the size, but just try some until you find one that fits. Then glue it in place just over the Holga shutter button.
As for exposures, it's gonna be about the same as any night photography- you can meter some stuff, but experience and note taking will be your best guides. Once you get the hang of it, most night photographers (okay, at least I do) find it easier just to judge exposures, than relying on usually difficult and inaccurate meters. To begin with, you will want to bracket heavily- take a guess and double that to begin with, it's usually very hard to
overexpose night photos.
Also, I'd recommend Fuji Acros for B/W film- very easy reciprocity characteristics to deal with- no adjustment up to 120 seconds, then 1/2 stop after that. For color, go with Provia 100, also very forgiving reciprocity characteristics (no adjustment up to 128 seconds).
Hope that helps a bit.
-Jason