I grew up playing tennis and did not play squash until later in life. I had pretty much given up tennis though for the last few years as I really enjoyed squash more. I have played a tennis though a couple of times in the last few weeks and have really enjoyed it.
I have been playing squash now continuously for several years without any real break. After one slightly frustrating game of squash I decided to go outside and pickup a tennis racquet. It was a very nice change of pace. My expectation was low and just went out to relax and have fun - and I did. Playing a game just to enjoy myself was a nice change of pace. I always play squash fairly seriously but when I went out to play tennis I was just hitting shots and not worrying about the outcome of a match. After a bit when I started to find my shots again I was hitting really well. I found that the more relaxed I was when hitting my shots the better the outcome.
The other thing I found really fun was volleying! It took a few minutes to get used to the pace of the ball but once I did I really enjoyed the challenge of trying to pickup whether the balls was going straight or cross court and then cutting it off and attacking it. Playing a firm volley away from my opponent in to a corner was great. Also trying to take the pace off and use soft hands to play a delicate volley drop was great.
After playing I felt great and took a step back from it and thought about what I liked about it so much. It was that I was relaxed and was playing the game to hit the best shots I could. I then thought about what I was doing well on the tennis court and how it translated on to the squash court. Playing relaxed certainly is helpful. Volleying the ball at the net is very similar to volleying the ball at the 'T'. Volleying the ball away from your opponent and in to a corner where your opponent is not is definitely effective in both. Also cutting the ball off and playing a nice soft touch drop shot is great in both tennis and squash.
Certainly there are differences in both sports and you probably should not be playing tennis right before a big squash match but as a change of pace it can be good for your game.