I just signed up for the site and posted this in my introduction but guess it is more appropriate here:
Old Guy, but in fairly decent shape (I think), just getting back into biking. Have been riding recently with my 46 yo son on his spare mountain bike. Mostly roads, paved bike trails, and few dirt/rock trails. Didn't really like the trails with a lot of rocks/roots so just ordered a Trek 8.6DS with intend of also doing some light off road trails. Just signed up here to hopefully get some info. on upgrades for the bike, like seat and pedals, and recommendations for a computer. With this type of bike is a computer with cadence overkill? I was looking on line and saw the Sigma BC 16.12 STS Cadence Double Wireless that looked like it had a lot of functions and is on sale with $29 off MSRP. With Black Friday coming up there should be some sales on accessories so wanted to start looking for upgrades so any advice would be appreciated.

Hello, just signed up myself and your post caught my eye as I also have an 8.6 DS, could not be any happier with it.
For a computer, I got the Sigma BC 12.12 STS as I feel that cadence is too much information, especially as you get to know your bike.
For pedals, I started with Shimano PD-M324 dual platforms as it was my first experience with SPD. Found the dual-sided flexibility a help
as I got accustomed to SPD, but soon found myself staying clipped in at all times and got tired of fumbling around to find the right side of the pedal.
I have since upgraded to Shimano XTR-PD M985 and love them. Pricey but worth it in my opinion.
For a seat, I would not change. Hope you enjoy your new bike. Here (I hope) is a picture of mine:

I would ride the bike for at least a couple of months before I even started to think about what to change or upgrade.

2 pedal camps : a broad platform pedal with grippy features like little pin-setscrews.


Or clip-less shoe & pedal combinations.

My butt has had its saddle preference over the years, but i'm not You..

Have your Trek dealer show you some options .. Ours will sell another saddle other than the one included,

and if that does not work [& you havent crashed , so resellable] exchange it

for a different shaped one of equal price.

Cadence can be nice to know for certain things.
I rigged up a cheap cadence meter while testing different length cranks/gearing combinations, since I thought having some "real numbers" to mix in with my "perceived results" would be useful.
https://www.instructables.com/id/Very...-Your-Bike-12/

I haven't found the need since, although all my riding is on pavement.
I did confirm that 165mm cranks worked far better for my bad knee, than longer ones.
Once you know what a specific cadence feels like, you pretty much know if you are reaching it or not without a computer.
My knees & lungs pretty much tell me if I'm where I want to be and my knee definitely means I won't be exceeding it. Sometimes the lungs say "time to back off" for a bit. (emphysema)