Remember my 401k does not invest in public funds -- but is limited to private funds ONLY available to 401k participants. No public fund family would allow daily trading in and out of funds with no penalties IMHO.
Bandito, You may want to check out fund families like Rydex or Profunds that are more geared toward holding positions for that kind of time span.
Be sure to check for early exit penalties for any fund you look at. And even if there's no penalty, that doesn't mean the fund will like you if you do a ST trade. I was banned from T. Rowe Price for making a ST trade in a no-penalty fund. (They claimed I was a market timer!)
The following three fund families allow unlimited trading in and out. I don't know if any 401k administrator would okay using them, because they are really intended for traders, but it can't hurt to ask, right?
ProFunds 49 different funds, including sector, international, and leveraged. http://www.profunds.com/
News 
Market Data 
Discover 
AI
