Strava is a good phone app that will track miles, speed, heart rate (with the strap), climbing, and it tracks your progress on user defined segments so you can gauge your progress. This is all free, but you can upgrade for around $60/year to get more in-depth analysis on your rides and fitness level. You can just start the app and stash the phone in your jersey pocket. You won't have immediate feedback, but unless you have the strap, or want to see how fast you can get cranking on a downhill, you may not need/want any info while you are riding.
If you want a computer, I have a spare Garmin Edge 25 and is about the size of a book of matches that will link to your Strava account and gives you the choice of the data you want to see, like speed, distance and HR, on one page. I think you can have it auto-scroll through other pages if you want more info than can fit on one screen. https://buy.garmin.com/en-US/US/p/508489 $75 and its yours. I know you are riding with the spouse and this really doesn't work for multiple users on the same device but it will get you started to see if you want a more robust computer. The Edge 520/+ are great and are smaller than a deck of cards, but start at $250 new.