You can use sound control to do nearly anything you want. Instead of moving servo motors, you can use a sound to turn NeoPixels on or off. You can train your Circuit Playground to recognize your fire alarm and communicate via USB to the Internet to alert your mobile phone.
Using a wired controller instead of sound is a low cost, tried and true control method. You should have a keypad or buttons mounted on something like a small box (maybe a mint tin). Find a wire that has several conductors, one that has the number of buttons plus one for a common ground. Flexible wire (stranded) is better for any twists and turns. The disadvantage to a wired controller is twisting of the wire and the length of the wire needed.
A more advanced control system could add a Wi-Fi or Bluetooth radio board. Pins #2 and #3 would be good for a software serial connection as they are free and do not conflict with using Pins #0 and #1 (which are shared with the USB port). You may need bigger wheels than the 3.75" / 9.5 mm size shown.
If you find the movements of the wheels are not quite what you would like (pivot on turn rather than a gradual turn), you can look to change motor angles for turn left and turn right.
You can move up to a real robotics frame rather than bolting the two servo motors together. Adafruit sells great frames (of course!) as do specialty robotics shops. This will add some cost. You can build your own robotics frame out of nearly any material. Think of materials that are sturdy and offer flexibility in attachments and drilling holes. If you have an erector set from a yard sale or are good in woodwork, you're in great shape. If you build a larger robot, I highly suggest a larger battery such as a 4xAA battery pack.
I suggest making your Circuit Playground removable. I am very much in favor of being able to take the main parts of a project (and the Circuit Playground is the heart and soul here) and reusing or borrow if I have a new idea. Teachers will also appreciate projects that do not take hours to disassemble. Use of a non-permanent adhesive such as Blu Tack or similar is one of the best methods I have found.