There are loads of ways to get involved in the work of the Kingdom, both domestically and internationally. Below are some links to various organizations who offer some solidly holistic mission trips for teens and adults. Under that, there is also a list of gaming and nerd conventions for your convenience.
Game Church: Predominantly evangelizing in a non-weird way at gaming/nerd conventions.
Next Step: Week long mission trips throughout the US and Caribbean 8BitKingdom went on their Nassau trip in 2012 and it was awesome!
Young Life Expeditions: Combining the youth loving spirit of Young Life with service throughout Africa, Asia, Europe, and the Americas.
Compassion International: Sponsor a kiddo in a developing or Third World country and then have the opportunity to meet them face to face.