Today, Sony announced a deal that gets you $20 back in credit if you spend $100 on PSN in October. It's the perfect time to put such an offer out, as it interacts nicely with the Day 1 Digital deal. Buy two games from that program (some of which are already 10% off for PS Plus) and there's your $100. You can then put that $20 toward, say, Spelunker HD. Or anything else on PSN.
The credit will be emailed to you in November, and sent to you as an XMB message if you're using a PS3.