Best value things that have increased your enjoyment of cycling

Garmin Dakota 20, found it cheap on ebay, rechargable batteries via micro usb, or if they run out, you can remove them and chuck in 2x AAs. It's fine if a car drives over it, but a lorry will write it off unfortunately.

Not blazingly fast but will do the job. The only significant downside is that the bike mounts seem to work well for a few months, and then lose their gripping ability.