Do you have a smartphone? For what you're looking to get out of it, a cycling computer app may be all you need. I've tested mine against a wired computer, and the accuracy of the GPS on the phone was very comparable. I've only had it fail once in the last 3 months.
Many people use Strava, and myself, I use Endomondo. It easily gives you average speed, top speed/low speed, lap times, distance, elevation, and even calorie estimates. You can see your route on a Google map, and it can highlight speed on the map itself. You can see the info on your phone or online. You have several data graph options on the website. And unless you want to be looking at the screen the whole time you're riding, the battery drain is minimal. I keep mine in my bag with the audible alerts on, and I can hear it give my distance and lap-time updates every few minutes while I'm riding.
Since my data needs are minimal, like yours, and I always ride with my phone anyway, this solution works really well...