Step 0: Define your target audience.
If you've talked to your LUG, you've probably not tapped anyone that hasn't heard of Ubuntu. Try to reach outward, far beyond the traditional LUG crowd. Try to define and reach an audience that has no idea what Ubuntu is. That's where the real progress is made.
Step 1: Advertise your event at least 1 week in advance.
Create small posters and place them in busy public places. Say something like: "We're getting together to talk about Ubuntu. Please join us!" (Have an email address or an RSVP link on the poster where people can indicate they are coming. RSVP'ing is better because it potentially lets you poll them for "What do you expect from this meeting?" )
Step 2: Look at the Responses
How many people are coming? What are they looking for? Can you meet their expectations?
Step 3: Prepare some small tent signs for the coffee shop tables. Make them professional, and make sure they conform to the Ubuntu Branding Guidelines. (You are representing Ubuntu so it's important to get this right.) Recognize that there will be many people that see the signs that have never heard of Ubuntu. First impressions count.
Step 4: Don't get hung up on convention/tradition/governance or any other construct that doesn't necessarily apply to your situation. The people you'd meet at an Ubuntu themed event might be totally different that what has traditionally been the Ubuntu LoCo crowd in other areas. Do what seems right for your community, culture and situation.
Step 5: Have fun. Always!
People won't stick around if the events are boring, dull, or if it's all about creating work. That's what day jobs are for! (Unless they are Ubuntu day jobs of course!)