I would personally pay off the loan from the parents. I'm assuming he's not charging you interest (good for you, bad for him), so he's effectively taking opportunity cost equal to whatever he'd be investing it in ie: he's clearly losing money on the deal.
Beyond that, I'd vote for contribute max to your 401k (as much as they'll match), as velocistar237 suggests. The rest goes into an after tax investment account (Vanguard, for me) in which you purchase based on your preferred asset allocation. Mine is 80/20 stocks/bonds.
Assuming 20,000USD to invest:
VBINX (60/40 stocks/bonds): 10000
VTSMX (100% stocks): 10000
Works out to:
10000 * .6 + 10000 = 16000 stocks
10000 * .4 = 4000 bonds
which is right at 80/20. VBINX also auto-balances for you, so you have less to worry about on a day-to-day basis.