Here’s a simple 80 lengths / 2000m swim session I’m planning on doing.  It should take me around 40-45 minutes to do I think.

Simplicity is the key here: easy to remember, easy to count (see here for gadgets to help you count lengths), easy to do:

Front Crawl Pull is swimming using just your arms (without kicking your legs.)
Front Crawl Swim is swimming using your arms and legs.
Front Crawl Weighted is swimming with 1kg weights on my wrists (!!)
(see image)