The list of troubles with adapting one's body to the void of space are numerous. Cold and radiation are the easy parts. In order for a body to be fully-buffered against vacuum requires either a robot body or a total re-arrangement of the fundamental principles of how the human body functions. Void Body modifications are changes that whole subcultures undergo en masse in order to be able to afford them. They allow a body to survive effectively in a vacuum through a host of accomodations. They aren't the choice for the super-advanced Civilizations, but for lower techs these modifications provide an early reach into the vacuum of space.
The first major problem is respiration, specifically gas pressure. While it's easy to be able to hold your breath for really long periods of time, if you try it in a vacuum your tissues will rupture from the extreme negativ epressure the arrangement will cause. This is to say nothing of the myriad gas pockets in the body, most especially in the gastrointestinal system. The correction to this is the total overhaul of the superficial tissues. The outermost layer of flesh is a sort of biolaminate that organically seals, but is air-permeable. Thus, it will evacuate of air and seal to the contours of the body. Underneath it is a layer of semi-hard cartilage plates overlapping, creating a "scaffold" that will prevent the body from over-expansion (or over-compression). The normal skin and other tissues then go underneath it. These external tissues do have nerve endings, thus the body is still able to feel.
The eyes have to be totally redesigned. Instead of "gel balls", they are hard cartilage spheres filled with an extremely thick, viscous material (one not prone to expansion). They are also wedged into the eye sockets more completely. Instead of an iris, the eyes have two polarized corneas - to adapt to light levels, one cornea rotates, adjusting the relative polarities to adjust light levels entering the eyes. The gastrointestinal system is a difficult one, and it merely is designed to wholly seal when the body is in void. Since it is also impossible for the body to functionally eat or drink while in void, this isn't so much of an issue.
The next major trouble is circulation. This is where the most significant redesigns of the human body come into being. The musculature of the body is radically overhauled - the body now no longer has a heart. Instead, the body's circulation is provided by the complete musculature, moving it along through discrete muscular action. If a bleeding injury occurs, the muscles immediately seal the blood vessel shut. While this will cause further tissue damage, it will prevent the massive loss of blood, and the blood will flow evenly, effectively, and quickly through the body despite a greatly reduced blood pressure. The blood is also made moderately adhesive, bonding ot the organic bonding materials in the external skin. It is designed to hypercoagulate, but not into a classic clot. Instead, in a few moments, it fiberizes, similar to what happens during psoriasis of the liver but in long fibers that mesh, forming a "weave" over the wound to seal it shut and return it to its normal vacuum-resistant state.
This, coupled with increased oxygen efficiency, allows a single human to survive unaided in a vacuum for an hour or more (depending on StringTech) on a single breath of air. Even an injury will fail to cause any more grief than the normal injury itself normally would (while a massive trauma would breech the vacuum seal, massive trauma generally causes serious complications even in an atmosphere and so needn't have special game rules attached to it). Coupled with the ability to resist cold and radiation, this makes the character able to go on spacewalk with only a maneuver module and no space-suit. Note that space is still a gravity-free vacuum, and so some form of maneuvering mechanism will be needed in order to move around functionally. Also, keep in mind that to "recharge" after a spacewalk requires more than a breath of fresh air - the character must have a meal or otherwise take in nutrition to replenish what is lost by utilizing the full power of the hyperefficiency of their respiration.
Tech Level: BioTech 6, StringTech 4 (or higher for longer durations of spacewalk)
Cost: Prohibitively expensive.