Believe it or not, we just spent $40 on rechargable batteries, so we have plenty of them. We also just bought a new receiver about 3 months ago.
I do have a couple ideas, but I'm not particularly fond of any of them.
I thought about getting a couple speakers so we could upgrade to 7.1 from 5.1. That's an option, but we actually have a couple speakers downstairs that we could use. Also, I don't really want to get 2 unmatching speakers and have a 5 of one type and 2 of another. Minor thing really, but that's me. Also, for only $300, I doubt I could get a decent 7.1 speaker system.
Another option is a 4gb memory card for my PSP. My PSP has a custom firmware so I can play "un-regulation" games, so the extra space would be nice. I also gave away my 1gb card (long story), so at the moment, I only have the original 32mb card.
I also thought about saving it up to get a new TV, like the Sony 46" XBR4 that I've been eyeing. Of course, then problem is that those $300 in gift cards only cover the first 10%. Also, I already have a 42" 1080p TV, so the only real reasons to upgrade are the additional 4 inches, the 1.3 HDMI ports, and the 120hz. Besides, I don't exactly have another $2700 laying around to spend on a TV... especially not after spending $1500 on a receiver a few months ago.
I thought about buying a video card, like a GeForce 8800 GT, but one of my computers is a couple years ago, and has a single-core CPU in it (the AMD FX-55) with 2 6800 Ultra's in SLI. The other has a Pentium D with a couple GeForce 7900 GS's in SLI. I'm not sure if a new video card is really going to improve game performance that much, and even if it does, I'm going to be getting a new computer sometime in the next year... probably after the new generation of video cards come out. So basically, what games would I possibly want to play between now and then that would benefit $300 worth from buying a new video card, and can I wait 9 months or so to play those games? Most likely, I can wait (given that the main game I want to play right now is Crysis, and an 8800 GT won't play that at 1920x1080 or 1600x1200 at good framerates). (For reference, the new computer will probably have the new NVidia card, plus an Q9550 or QX9770 or something in it, so it should be able to play everything for a while.)
I have this problem where I always want to spend the gift cards I have. I always tend to spend them on something, whether it turns out to be a good purchase or not. If I wait to buy good games when they come out, I'll end up spending them on shitty games that I wouldn't have purchased otherwise.