I'd prefer a combination of giveaways and longer running contests.
The giveaways could just be simple random "leave a comment", "identify this widget" or perhaps something like picture/video submissions; take a pic of your workbench, inside an old/unsual piece of electronics, best dave jones impersonation, etc.
For contests i think a few shorter ones that focus on doing a simple task or making something novel within specific constraints "No ICs/MCUs", "What can you do with XXXX".
And perhaps a longer more open-ended contest, such as creating a Project that is resonably inexpensive and would be useful for a beginner/hobbyist. Like a function generator, LED tester, power supply, etc. something similar to what you find published in an electronics magazine. Train of thought is basically even though a beginner may not have a chance at winning they still benefit from the contest. (a nice $400 multimeter or 'scope should be a good incentive to get some quality submissions.)