To quickly change the Spike into an Explorer robot, you can strip legs, claws and tail, leaving the chassis as a basis to build the Explorer robot.
Then add wheels, a bumper with touch sensor and an ultrasonic sensor mounted on a motor.
Strip legs, claws and tail
Add two wheels to the motors
Make rear wheel
Follow step 7-9 of the Castor bot building guide to make the small wheel. Then add it to the rear:
Add bumper with touch sensor
You may use your imagination, added with the instructions for the Bumper car and the examples below to add a bumper.
Add motor on top with ultrasonic sensor
The motor will be mounted in such a way, that it can be quickly removed.
Mount the ultrasonic sensor. Replace the axle with a slightly longer one, and add an extra pin to ensure a firm connection to the motor (the sensor will rotate, so it must be connected well):
Connect the cables. Extend the cable if necessary with a extra set of female-female jumper cables:
(how to connect the sensor to the servo-headers of the shield was explained here)