Our household has 4 vehicles, a whole-house generator, 2 lawn tractors, a log splitter, a backhoe, etc... I do 99.99% of the repairs and maintenance on all of them. Without some sort of documentation, I could never keep track of it all.
I use mycarprojects.net to keep track of everything. There are columns in which you list the service performed, the mileage at the time, the date, and the cost. At the bottom of the page, these costs automatically add for you, so you can see at a glance how much was spent during its lifetime. This has served me very well indeed when I sell one. For every type of service performed, you can choose to create alerts to remind you for upcoming services required. These can be set for mileage, time, or both. There is also a comments section for each entry to make a note indicating what brand, part number, size, viscosity, etc... was used. In the master notes section I keep all VIN numbers, which are frequently needed when I order parts. There is also a mobile app I find very useful, which can read from/write to the database and also calculate (and store) fuel economy numbers. It is a terrific app and website. Best of all... it's completely free!
I cut-and-pasted an entry from my last service to give an example of what is there. It is not neat here, but is in neat columns on both the website and app.
Replace Brake Hoses 166,261 6/6/2016 $70.00 Dorman H620950 (LF), Dorman H620951 (RF), Dorman H620964 (LR & RR).