A perfectly generic cyclocomputer will measure ride time, distance, total distance since installation, top speed and average speed. Should fulfil most of your needs.
I use a heart rate monitor/cyclocomputer hybrid(Polar CS300, there are others probably equal). This also calculates calories and has a cadence counter.
Not "required" by any means, but does offer some more data on how hard you're working out and how you're doing.
Cadence is useful for me 'cause I've got wimpy knees and a history of mashing/crude pedalling technique.
Calorie counter is useful as it can provide some ballpark values of how much energy you're actually using up.
Be careful though, getting one of those can seriously limit the amount of "treats" you can reward yourself with after a ride ;-)