Help me design my summer workout(s) plans

Maybe run a nice, easy recovery run on Sat. since you've played tennis that morning. Something just to warm up the muscles. Then run Sun., Wed., Friday.

Weights could be M/W/F, so you would have to double up a bit. Do you have time to do one workout in the am, the other after work?

Gaining that much muscle is going to be hard unless you left 4 days a week. That's going to cut into how much running and tennis you can do because you'll be too sore.

If I were you, I'd run 4 days a week (2 easy, 1 tempo and 1 longish run), lift 3 days a week (2 full body days and 1 day focusing on your weak areas) and play tennis 3. I'll leave it up to you to figure out which days to do each.

I'm in the same boat, just trying to crawl back into the milieu after an extremely long absence, and I've decided I love lifting at least as much as running so I am not giving that up. I'm also trying to throw a swim or two into the mix, which for all intents and purposes would be the same as your tennis as far as the workaround.
I was thinking of the 4-day a week plan too, although I am trying to see if I can squeeze in four lift days. Right now I think three is all I can manage. Since runners develop stronger lower bodies anyway, my personal plan is two upper body days and one lower body day, for now. It is going to require a couple doubles though, if you have the time. Tennis is good for all over and you'll hit the upper body more than you would just running, so it's really a matter of where you want to see the definition as to whether you do 2 of one and one of the other. I personally don't like full body days and rarely do them anymore. I like to concentrate on one or the other region (upper/lower) and I also do combination exercises and pyramids to get a better workout in a more concise timeframe.
For example: a curl-and-press will hit the shoulders, biceps and secondarily, the triceps so to me three sets of that is more effective if I am time-crunched than doing each individual exercise. The neat thing about weightlifting is that you can really make it flexible and I love the idea of not just isolating one muscle, but hitting a group of them at the same time. I assume you know what pyramids are? I like those too because they can build muscle if that's what you're going for, and again, you get a great workout with the increased weights because (IMHO) you challenge the muscles more by upping them with each set.
I'd prolly go with PH's plan, although I'd do the upper body lifting twice rather than a total body workout. But again, where are your weak areas and what are you hoping to achieve goal-wise as far as increased strength and muscle mass? And how much time during your day do you have to work out, i.e. mornings AND lunch AND after work? If you're really flexible you can get it all in. If your workout time is limited, then you may have more of a challenge to balance.

Good stuff genie and PH. Looks like I'm going to have surgery on my knee next month, so running is out for the time being. Tennis will be my only real cardio. I'm just going to do an all over body workout every other day and work in my tennis as apporopriate. some days will be doubles (tennis and weight training) and some days will be empty.

I will have to rearrange the workout schedule yet again once I start post-surgery rehab. :(