Ejected out of an airlock. NASA did some vacuum tests on large animals (cows, sheep etc) and discovered that it typically caused death within 30 seconds to a minute.
Generally your blood vessels rupture where they are close to the surface, mainly the lining of the lungs but also eyes, intestines etc. You do not have time to get cold. You will get cold much faster in a bath of ice water than you will in a vaccum.
In D&D terms I would probably go for Con damage rather than hp. You could make a fort save initially (getting harder each round) and after the first failure take d3 con / round.
Planet explodes. Most of the debris will have the orbital momentum of the original planet so the bulk of it will disperse along it's original orbit as a new asteroid belt. Obviously it depends how violent and total the explosion is. You are sure to increase the likelyhood of metors crossing the orbits of other planets in the system for a long time to come. It actually takes more energy to stop a planet in it's orbit (and drop it into the sun) than it does to totally vaporise it, so I wouldn't expect huge planetbuster chunks to cross other planets orbits. Still a 10 mile across dinosaur killer size rock would be quite reasonable. Remember that the solar system is mostly empty space though and a direct hit by anything big in the short term would be quite a fluke.
All planets have a gravitational effect on the others, but for a planet like mars it would be very minor compared to the pull of the sun. Orbital changes are going to be more of a long term (thousands/millions of years) to be anything significant.
I guess if the planet had a decent size moon that was intact, it might deviate quite a bit from it's orbital path, depending which direction it was headed when the planet blew.
