How to add activity on Forerunner 235

You can't, full stop.

You simply cannot do so on a Forerunner‑230 or 235.

The closest you can get to it is by developing and/or downloading and installing a Connect‑IQ app that tracks timed activities and produces FIT files for them in a particular way. Such Connect‑IQ apps will be available to you to execute instead of the factory-installed activity profiles such as Bike Indoor or Run.

Connect IQ apps are the way new things can be added.

Here's one for both Hiking and Outdoor Walking:

https://apps.garmin.com/en-US/apps/39d3b5e8-67db-4517-93a8-5ec485269635

It actually shows you more data than a native app can. And it's free!

The Hike app ^ is great! As for swimming, I use the Other (GPS off) for my indoor lap swimming. Later you can re-categorize it to Swimming and edit the activity with stuff like pool length and number of laps which give some (minimal) data about your swim ... not ideal, but at least you can log your swim activity, distance, calories etc ...

Max

A great app for pool swimming is "Pool Swim" by daniel27. I've been using it on my 235 and it works well. Just pick the length of the pool and it'll keep lap count, stroke rate, HR from the optical HRM, etc. https://apps.garmin.com/en-US/apps/f134a024-6ee4-47d3-8aec-3a397e3c9733

thanks! finally got one way for tracking the hiking option. this is great! 

I tried to use some apps from Connect IQ, however, they are not free and also some trail version which sucks!