One thing about heaven, it won't be forever...
What we know of heaven indicates that it is outside this speace/time contuum. There isn't really actual matter, or energy, or anything there, nor is there time, since time is a product of matter. God will supply an illusion of such for people who get there, so that isn't a problem (although without there being time it may seem slightly wierd at first). However, God specifically mentioned that when he comes back to earth, there would be a "new heavens and a new earth". The word heavens, plural, is the word for planets and stars and such in this physical universe. We will thus be in a newly made, physical universe, which being physical, will have time. We will thus be able to change and grow, and are being prepared now to better appreciate that and interact in that with the people who also made it there. The new universe, since God the creater is present, may also change and grow. Since this will go on forever, it is no wonder God says that now we cannot understand it, since both we and it may change a lot over eternity. Speculating on it is above my pay grade, except to say that it won't be boring.
of course, some would immediatly jump on this idea and say that we will evolve into Gods. Well, even if we got twice as powerfull and twice as smart every second for a million years, our power and smartness compared to Gods sould still be the exact same ratio as now, finite compared to infinite. besides, I kinda doubt anything like that would happen, since I doubt we could handle it, God will know what we can handle and do that.
Oh, and a physical universe can have physical cars. To start with, that is, I imagine that, after cars, soemthing better will come along, and then something better yet...this week, muscle cars
, next week, rocket cars
, the week after that
.