The first thing that you need to do is plan for any type of weather as it could be very warm or very cold... and both during your time spent on the Haleakala Crater. If you can possibly make it in time for sunrise, it would be spectacular, but unfortunately, we didn't realize how far the drive was (even when you are close as it's a winding back and forth drive up to the top). You should allow about another 1 hour extra for slow drivers and you want to get to the summit about 20-30 minutes before sunrise.
The summit is possibly different that you imagined as it is not a typical volcano as the whole are is much larger than you expect it to be. You must be in good hiking shape as when you go down the trails, you must return uphill and it's a never ending uphill experience. The air is very thin, too.
You can appreciate the vastness of the area by viewing the top photograph and seeing how small the people are in the photograph. If you are hiking in the summer months, you should see several Silverswords in full bloom (which is only found on Haleakala).
It is a place that I will never forget as it was spectacular hike.