Hmm, I think the thread is intersting.
I'd best start it off with Carke's third law: "Any sufficiently advanced technology is indistinguishable from magic."
Thus I wouldn't even bother with advancements based on state of the art science, as it has some disadvantages: firstly, it's quite a lot to research if you want to be accurate. Secondly, no matter how much you research it, you have to expand upon it in a fantastical way, which is hard to pull off without getting rediculous.
So rather than scientifically back it up, chose things they are capable of and don't ever try to explain them. Just look at what Lucas did with the Force. With one sentence he degraded a mystical power to a benevolent infection. Let your space travelling be godlike, unfathomable in powers, and define them rather by morality than by technology. Given the advantage they have, how would they cope with the problems of earth? Would the intervene, or observe? etc.
Also, explore the impact on humankind, economics and religion.
And as a small sidenote on flying cars and timetravel: While the first one is one of the tropes of futurism, I don't think we will ever see flight as a indivudial way of travel for the masses. Most people can barely cope with the two dimensions of ground level traffic, so better not add a third one. Also, as flight takes much more energy than driving, it won't happen as long as we haven't found a more efficient way to generate and store it than via oil.
And I don't think that unlimited time travel is possible for one reason: we would've met time travellers or at least found artifacts of them. Because even if time travel was limited to a small elite, it still would cause myriads of time travellers over time being able to travel to the past, and, even if they are guided by a common directive of stealth, somebody is ought botch it up sometime (murphy's law), thus we'd have some sort of proof of time travellers.
The only way I think time travel would be possible would be like in a corridor: So if a time machine was invented, time travel would be possible from the point where it was turned on until the point it was turned off.