The Sony Smartband SWR10 is fantastic value at £19.95. Coupled with the Lifelog Android application, you can log your activities throughout the day. You can track lots of different events that occur during your day and night. Log your sleep pattern, calorie expenditure, photo taking, music as well as pedometer functions. See here for more information on the Sony Smartband SWR10's capabilities. You will need to be running at least Android 4.4 to use the device with your Android phone. Unfortunately iOS and Windows based phones are not compatible with it. Reviews are positive and i have to say that for £19.95, i think it's a no brainer. Especially if you're starting out and want to give this type of wearable technology a go without spending a small fortune.
Thanks to rucra27 @HUKD for the tip!