At HubPages you can write about anything and everything except adult content and other prohibited stuff (you can check this by reading the HubPages policy.) Such content is against the HubPages as well as Google Adsense TOS.
Its best to write about what you know because original and unique content attracts readers and you are always confident and sure of yourself too. You can write about your hobbies, travel stories, household tips, movie reveiws, money making strategies, review technology products or share your knowledge on a topic like biology, gardening or astronomy or even share your personal experiences.
You can always get ideas from the idea bank if you like. There are loads of ideas so just be yourself and let the world know what you have to share.
Good Luck! Cheers!