Utgardloki
Mongoose
I was thinking it might be easier to modify a character's effective strength then to modify the weight of everything. This avoids the problem of making characters too light in a low gravity world, although there probably should be a limit to how high a character's effective strength gets because eventually people become able to move around things so massive that moving them around at all is imperceptibly slow (F = ma rule). Also, the modified strength would only apply to some situations (such as jumping and lifting) and not to others (not to breaking things or doing damage).
So a character on Mars could get +10 to effective strength for lifting and some athletics checks. A character in an acceleration field might have -15 (and if this drops you below 3, you can't move). (I'm not sure how to punish large creatures who would be even worse in a high-G field.) The maximum increase might be +50 in a zero-G field (although perhaps some rules might be required for the difficulty in starting and stopping massive objects).
So a character on Mars could get +10 to effective strength for lifting and some athletics checks. A character in an acceleration field might have -15 (and if this drops you below 3, you can't move). (I'm not sure how to punish large creatures who would be even worse in a high-G field.) The maximum increase might be +50 in a zero-G field (although perhaps some rules might be required for the difficulty in starting and stopping massive objects).