I haven't been biking very long but there's a piece of gear that I have that is valuable with two of my long time hobbies (very long range competition shooting and flying) that may be of significant use in biking. That would be a Kestral 4000 weather tracker. It will measure altitude, wind speed, humidity, barometric pressure, density altitude, dew point, wind chill, heat index and several other even more esoteric things. More importantly it will take and store the data at intervals and either show the raw data or graph it. It's about the size of a pre-flip cell phone, weighs a couple of oz and cost around $250. For all the altitude/density data, you'll need to do an initial calibration on it from a point of known altitude or barometric pressure.

I know it's not needed by everyone but there may be someone who will benefit. I turned it on this am and got freaked out since the heat index was 112 F at 10:30 AM here in Florida. Lord, give me a break.