There are a bunch of them, but what you need to do is think about what operating system you want on it. Apple iOS? Then get an iPad. Android? then get an Android tablet with good hardware, like a Samsung Galaxy.
The majority of them are Android based. I would stay away from Blackberry Playbook, bad reviews and OS, and no updates until February. You can get Amazon's tablet, but I wouln't...it's a hacked version of Android. Wait a little bit if you like Android, until a tablet is released with ICS "Ice Cream Sandwich" Android version, optimized for Tablets.
You don't need to have a data plan with a tablet, just tether it with an app, like you said.
I would also wait until the day after Thanksgiving, Black Friday...you should be able to pick a good one up between $250 - $350.
Keep an eye on this page as we get closer to Black friday, it will list the sale events for tablets... http://blackfriday.gottadeal.com/BlackFridaySales/Category/Tablets