The best upgrade I've ever done to my T420 was a 65W GaN charger (less than $20) and a USB-C to barrel power cable from AliExpress (~$5). So much more compact a

The best upgrade I've ever done to my T420 was a 65W GaN charger (less than $20) and a USB-C to barrel power cable from AliExpress (~$5). So much more compact and aesthetically pleasing than the stock charger. Another cool upgrade is an HDD bay that you can install in place of the optical drive. Costs less than $5, lets you install both regular SATA SSD (no need to look for mSATA) and an HDD. Edit: total cost of my T420 with all upgrades I've done over the years is around $150. Pretty neat for a daily driver. I'd probably still use it if I could get hands on a good display and a conversion board, but I've switched to an M1 MacBook Air since then. Maybe I'll do a frankenpad build with the M1 motherboard in a ThinkPad case though!