Normally you make simple rolls for basic actions against characteristics. I was thinking of allowing a bonus if a character has a related skill to such a feat. For example, jumping is Agility. A character with Acrobatics would get a +10%.

I was also thinking that some tests could easily fall under two characteristics, for example, again, jumping. Jumping could be Strength or Agility. I was thinking of allowing players to choose which of the two to use in such a test and apply the Characteristic Bonus of the non used characteristic to his or her % for success.

Thoughts?

I'm completely behind the former, as gm determined bonuses certainly apply to any and all checks - if a similar skill works, then grant them a bonus so long as that skill is nearly as good as jumping.

As for the second, not at all. If players are given the choice between which stats to use in given situations you will doubtlessly run into a min/maxer who realizes he need only increase one skill since he can choose to use it in place of another. I would stick with the idea that one is limited to the stat bonus stated, as this defends the idea that a person with different strengths is going to be better at different activities. No matter how much I'd like to, my strength will not allow me to walk a tight rope, the players should feel the same. These are the limitations they have been born with, if they wish to increase them, there are mechanics for that.

IH actually has a number of rules about one Skill aiding another (the Lores are the most obvious examples). Usually, it costs you to synergize - in the case of Lore skills, it takes longer to perform the primary Skill roll because you're going totally "by the numbers" on the task.

I think it is a good idea to allow a skill to synergize with other activities, altough I would consider requiring they at least have the +10 level before allowing it to apply as a way to really reward 'training'. I also agree that allowing choice is a gift to min/maxers.

I often do allow my players to choose the best of two skills. In theory it does allow for a degree of min-maxing, but they can't anticipate every situation - I make sure that every skill has uses that other skills can't duplicate.

Of course, I also have the advantage that none of my players are system-hackers