you could take some of that money, say, $20 and buy Google Adwords vouchers. I saw someone over at webhostingtalk.com selling those for around $5 for a 52$ (30 GBP) voucher. Look around a bit, maybe you can find more deals like this.
After that, look around for a few websites you'd like to advertise on and that have google ads on and click the 'advertise on this site' link. This way, you're sure to get a campaign more targeted than you would if you just went and selected a bunch of random keywords.
Hope this was helpful :tu:.