I’m not sure what your Chromecast options are if you don’t have a USB port.
There are a bunch of other options, depending on how much you want to spend, and what sort of functionality you need. If you DO have to buy something new, i don’t think i’d get another DVD player with a USB port. You’re better off getting something that offer functions that complement the blu-ray player you already have.
One relatively cheap option that will allow you to play files off a USB key is to get a Raspberry Pi, and install a version of the media player software XBMC, called RaspBMC. This set-up should cost you about 70 bucks, and it will play just about any video or audio file that you could put on a USB key.
One downside to this system is that it’s not just plug-and-play, like a WDTV or a Roku box. You’ll need to install the RaspBMC on an SD card, which then goes into the Raspbery Pi. I have one, and overall it works pretty well, and seems to have no trouble running 720p and 1080p video, and my version is one generation old so it doesn’t have quite as much memory and processing power as the new model. It plays MP4, MKV, AVI, ISO, etc., etc.
If you want to spend a bit more, you can get full Windows functionality in a small form factor computer like this one. While it is currently listed at over $300, i got the exact same model on special just over a month ago for $160. If you’re patient, you can get a good deal. At $160, it’s really not much more than a Blu-Ray player (which you already have), and it’s much more versatile than a set-top box like a WDTV or a Roku. Because it’s a fully functional computer, it’s also really easy to use web applications like YouTube, Netflix, TV station websites, etc., etc. You will need a keyboard/mouse combo, preferably wireless, to go with it.
Another option, somewhat similar to the WDTV, is a Roku box. It has a USB port, and will play quite a range of files, but i have encountered a few instances where it has trouble, especially if you encode your files with 5.1 sound. The video will sometimes play, but without sound. I’ve found that re-encoding those files with plain old stereo sound works. Roku also sometimes has a bit of trouble with the video in some MKV files, even though MKV is listed as a supported format.
All of the above options have HDMI outputs, so you can plug them straight into your TV.