• PMI's IS Specific Interest Group (SIG) has lots of free webinars (1 PDU per webinar) available to SIG members ($35/year to join).  This is a category 3.
• You can write an article on any area of expertise you have and submit it to your local chapter of PMI.  This is a category 2B and earns 15 PDUs.
• For each 12-month period you work as a Project Manager, you can claim 5 PDUs.  Over the course of three years, you can earn 15 PDUs.  This is a category 2H.
• You can claim up to 15 PDUs for category 2-SDL, Self Directed Learning.  This includes articles, books, instructional manuals, videos, CD-ROMs or other material resources.  Podcasts are a great way to earn 15 PDUs under this category.  Here are two popular ones: www.pm-podcast.com and http://www.controllingchaos.com
• Attend local PMI chapter meetings!  Many chapters have breakfast, lunch, and or dinner meetings that will give you 1+ PDUs under category 3 per event.  
• If you already volunteer for a non-profit organization, you can claim 5 PDUs per year.  Thus, you can get up to 15 PDUs under category 5 for giving of your time. 
• Last, you can take a "hot topic" course (Agile / Scrum, Six Sigma, ITIL/ITSM, etc.) that will earn you lots of PDUs and give you a certification that looks great on your resume.  Look for Registered Education Providers (R.E.P.s) with PMI to earn PDUs under category 3.  Here is an example of one for ITIL v3 Foundations certification that gives you 18 PDUs under category 3. 

Once you sign into PMI's website, you can submit PDUs online.  The PDU site has been recently redesigned and is really easy to use.  So, you now have ideas on a total of over 80 PDUs that you can get in a few weeks time.  No more excuses on maintaining your Project Management Professional (PMP) certification!