Thank you for a really good question! Yes, there are several exercises you can do to improve speed, flexibility, strength, and independence of your left-hand fingers, all without having an actual guitar on hand! Before we get started, here’s a quick warning - please stay away from the various mechanical contraptions that claim to help you build finger strength and improve speed - in the vast majority of cases they do not work and very often they lead to various injuries.
Understandably, it's quite difficult to accurately explain these exercises in written form, so I'm going to include a few videos that give you a very good idea on what you can do.
#1 - Hand Stretches for your left-hand fingers - These are going to help you strengthen and limber up your fingers and also minimize the risk of injury. Here’s a really useful video by the great Jon Petrucci of Dream Theater in which he outlines his own system of stretches and warmups. - https://www.youtube.com/watch?
#2 - “How to open up your fingers” - In the following video, I demonstrate what I consider to be the very best finger exercise to strengthen your fingers without a guitar - this video is part of my “Ask the Guitar Coach” video podcast that answers the most important questions about learning to play the guitar - Can fretting hand fingers "open up" with practice? https://www.youtube.com/watch?
I hope you’ve found this answer helpful - Best of luck on your new musical journey!