Congratulations - you're in a very enviable position compared to many startups - you already have an audience.

I would recommend that you open a dialog with your readers - it doesn't have to be obvious (it can be), but you should include them in the decision, and find out what they want.

You could take a direct approach and poll them. That might seem a bit uncomfortable, but if you believe in the value of what you'll offer them - and give them a say in that offering, it shouldn't.

You could also take an "offers" approach - where you test a few limited quantity, limited time offers (via email, banner, inline, etc) - this allows you to test the waters with multiple products before committing to a consistent offering. You can test products (physical, digital), services, affiliate offerings, and the list goes on.

What to offer is a hard question to answer without more details - but suffice it to say you should look at what has performed historically among the ads on your site - as that will give you insight on what has captured their interest in the past.

Something very important to remember - you've built this based on the value you add via the blog - don't stop doing that. I've seen more than one blog make a commercial attempt - and in doing so stop or greatly reduce their content after getting excited about initial conversions. Once they'd hit their entire initial population of users with product marketing, things slowed down, and then they had to play catchup on the content front to regain traffic.