• Use a shorter long-break.

Are you sure you need 30 minutes? If you're feeling that's too much time you can reduce it to the minimmum suggested of 15'.

• Look for activities in your to-do list

While you were working on your last pomodoros, what did you feel it was important doing and forgot to schedule? You're supposed to take notes on these activities under UNPLANNED&URGENT. Be it talking to a co-worker about a pending matter or scheduling an appointment at the dentist, this larger break is the perfect time to empty that list.

• Take a walk around the office

Not everyone can walk freely at work but if you can don't miss the opportunity. Find out what your co-workers are up to, you might get inspiring ideas by asking for opinions or just watching how they handle day-to-day problems. See if you can help doing some of their tasks, it'll earn you credits to ask for help in the future. Knowing your co-workers can help speed up group projects and strengthen your networking. You never know who might give that extra push in your career.

• Yes, take a nap.

Power naps are told to estimulate productivity. The ABC News reports - citing a NASA Study - that a 26 minutes nap can boost your productivity by up to 34%. The same article suggests that the ideal time to do it is from 1 pm - 3pm, a said natural time to feel an energy minidip.

From Pomodoro Technique v1.3

The 15-30 minute break is the ideal opportunity to tidy up your desk, take a trip to the coffee machine, listen to voice mail, check incoming emails, or simply rest and do breathing exercises or take a quick walk. The important thing is not to do anything complex, otherwise your mind won’t be able to reorganize and integrate what you’ve learned, and as a result you won’t be able to give the next Pomodoro your best effort. Obviously, during this break too you need to stop thinking about what you did during the last Pomodoros.

I always feel I could be doing something really productive with this time.

You are already being productive when you apply Pomodoro Techniques. That's the point. Read again top paragraph. You should do something in your 30 minutes break, yes, but never say "Oh my god, I'm doing Pomodoro Technique but I must be more productiv in my break times".Purpose of break time is refreshing your brain. So you should ask yourself;

Which activity helps me for a fresh brain?

I find that while knowledge work (which what most of us do, sitting at our computers) stagnates the body and tires the mind, exercising does exactly the opposite: tires the body and refreshes the mind. I get my best ideas while exercising, for instance, because my mind is off work, emptying, and creativity sets in.

I'd suggest you to do one of various activities that would in this way refresh the mind and foster creativity.

• Physical activity, as mentioned. I have actually taught myself juggling 3 balls during my 15min breaks (and after learning, I'd also often do it during my 5min ones--it's so fun!)
• Teach yourself touch-typing, if you haven't already. At first it might take a minor toll on the mind, but once you get going it's such a liberating physical practice, just letting your fingers run to the keys on autopilot, as if the control was in the hands and not the brain (called a motor-skill). I use Dvorak; I had first learnt QWERTY but I find the former much more relaxing as it's less of an effort to type. It's a good use of time too, doing data entry, for instance. Or getting the self-equivalent of a psychologist by doing a stream of consciousness emotional dump on a journal.
• Brainstorm, do diverging thinking on some project you have, like listing pros and cons or just blurting out plans. What David Allen would call a mind dump.
• Meditating, as suggested, is another great use of your time and refreshing as any. I learned it through 8 Minute Meditation, which is quite adequate for the breaks.
• Revise your productivty system (GTD, etc) or revise your project and goals list. I personally find it really refreshing and motivating to re-read and refactor my goals and vision, and my mission (purpose) as well. It's a quick birdseye view of your life that can both get you off the immediate work and remind you as to WHY you do it at all.

You can get quite creative with your longer breaks, but as suggested you can also shorten it do 15mins, I've done it and much prefer it.