Smart description, pre- and post-activity comments
The description, pre-, and post-activity comments fields could work with basic math functions and variables (eg. set FTP).
For example, I like to note "15+17+12=44 minutes of sweet spot intervals". The web app could do the math for me, just like Excel.
Another option is to check interval numbers against reference metrics like FTP or max HR. For example, I could write 30 min @ 300/FTP, and it would return the interval avg power/FTP ratio.