This is the computer that I use.
https://www.polarusa.com/Products/cs/cs200cad.asp?cat=consumer
It monitors heartrate, cadence, calories burned, etc. I've been monitoring my heartrate for years, really helps to keep me from pushing to hard on hills and is great for interval training. I didn't realize how important heart rate monitoring is until last year. I was training for Ride the Rockies and every time I got my heart rate up to my normal zones I started struggling with breathing. I thought I had lost my cardiovascular conditioning. I struggled all week on RTR and was even told by a friend it was all in my head, I was being a "whiner". On the last day of the ride I had a severe asthma attack and landed in the ER. Turns out my asthma wasn't under control and had I payed closer attention to my heart rate and checked with my dr. I may have avoided a trip to the ER. Now when I ride my heart rate can go as high as I want without gasping for air.
I'm another lightweight rider If I'm doing a short ride, under 20 miles I carry 2 water bottles, identification, a small amount of money, my inhaler, lip gloss, tube, pump, tire irons, a packable jacket and sunscreen.
I also carry a small lock, and use it even if I do a quick bathroom stop. We had a bike stolen because it was unlocked. Last summer a mtn bike ended up in our front yard, it never was claimed. My lock won't keep a serious thief from stealing my bike but it will keep some one from taking it for a "joy ride". I also keep a piece of paper with the serial no. of my bike in my jersey pocket or in my backpack. That way if it is stolen I can call it in immediately. All of our bike serial nos. are recorded on our computers. Our police dept won't talk to us about a stolen bike unless we have serial nos.
When I do long rides I use my Camelback, I drink more that way, and carry a lunch, peanut butter sandwich and a cliff bar.
My handlebars have my Polar computer, no room for anything else. I use a mirror that inserts into the end of the handlebar in place of the plug. Handy to have on the trails around here as many cyclists don't bother to call out when they are passing.
As for sunscreen use, at the age of 60 I'm seeing the signs of what unprotected sunning can do to your skin. My mother and grandmother had beautiful skin but they never went into the sun without covering up. I'm fortunate because I lived most of my life at a low elevation with lots of overcast days so I don't have the signs of aging that many of my friends here have. However, the Colorado sun is quickly changing things. I don't leave home without it.