Word for "player" without the negative connotations

Player only has negative connotations in the context of romantic human relationships. If the context of your piece establishes that a game or sport is being discussed, player would not ordinarily carry a negative connotation.

How about:

• participant as you've said
• participator
• contestant
• contender
• instrumentalist in the non-musical sense

I'm having trouble coming up with a noun that describes such a person, but it sounds like you might be describing someone who:

• has zest for life
• lives life to the fullest
• is not a benchwarmer
• doesn't sit on the sidelines
• is a real go-getter
• is a mover and shaker
• seizes the day
• is on a mission
• lives with gusto
• has pluck and verve
• is high-spirited & dauntless

Maybe some of those could help.

In the context of a competitive sport a player could be a sportsperson, athlete, competitor or a team player.