Well lets talk about our main sentient species. We can start with "how do they look"
So for comparison, we'll start with traits that make a thing sentient on your garden variety human.
Mentally, a human sentience is defined by self-realization, abstract thought including possible outcomes of actions, creative tool use, environment adaption, and the ability to act against obvious instincts.
Now these things can be expressed in a lot of ways. We are realizing now that other earth species may have these traits to greater or lesser extents. We should decide where our species' strengths lie, but remember that they must be able to meet the test of sentience (not that I figure this would be a problem). But we can also think of what kind of challenges this species would face mentally. Does their society suffer from things like depression, fear of death, DESIRE of death, fearfulness, fearLESSness, or exaggerated optimism? Obviously there will be individuals with their own unique problems, but if you look at human demographics you will see trends in populations. Also since we have humans to compare them with... how do they compare with humans?
Now Physically. Humans became the dominant species largely thanks to our hands. Anyone doubting that can try to live without them for a while :). Now there are people without hands that can do amazing things with their mouths and feet, but I'm sure if humanity started without hands... our mouths and feet would probably be WAY more remarkable by now. The bottom line is, our species must be able to manipulate things so they can construct their tools and use their abstract thought to influence the world. a hyper-intelligent shade of blue is a nice thought, but tell me how blue influences the world. Of course, for other comparisons, I would point toward cuttlefish, Taxxons (animorphs), elephants, amoeba, and electroconductive telekinetic crystalline entities. So... How should our species shape its world? Even if they're just psychic cubes, they would be able to mind control others into doing their bidding!
Once we have these, we should consider how the primitive members of this race faced the challenges of their day and what those challenges were! The bushmen of africa and the aborigines of australia have an amazing (though primitive) ability to cope with their surroundings and are capable of happy lives despite living in areas that might be considered inhospitable to some... the same way that early europeans dealt with living so near to the arctic circle or how the Philippine peoples managed to get across to islands. They adapted their behaviors and their tools to overcome being naked, weak, and unarmed. We as humans today can easily forget the struggles of our ancestry... after all, we weren't there. So, what did our species have to deal with? Predators? Deserts? Long Winters? Acid storms? Radiation spikes? Oceans? and don't forget, it doesn't have to be just one of these things! How did our critters get to be the dominant life form?
I think once we have these ideas in mind we can REALLY flesh these things out! Remember, in the creative and evolutionary process you can easily say "Form Follows Function" So lets get those functional questions answered. After that we can concentrate on what they might look like.