First and foremost? Health Care overhaul. Not only is it useful immediately, I’d expect there to be quite a few health-related issues when cannabis becomes legal.
As @johnpowell has said, drug rehab and education would be a close second.
This is assuming the economy is back on it’s feet by then; if not, I would expect the money would go straight into paying off the federal debt, as well the effects of the economic collapse (mainly, to pay off these two stimulus packages).
Renewable energy would also be very important – with that comes savings across the board and innovations in many aspects of society (i.e., power plant upgrades, newer vehicles that people would actually want to purchase, possible military use [fusion-powered BDUs and exoskeletons, OMG!] etc… the list goes on). With savings and innovation comes new business. That equals a boost in the economy.
I’d also fund science foundations heavily; stem cell laboratories (find a possible alternative to embryonic stem-cell use), free and stable national WiFi (freedom of information), feasible space-exploration possibilities (listen, we’re bound to need the ores in asteroiods one day), etc. Again, this would probably come in last on the priority scale.
Many, many things I’d use the money on.