Soul - It all depends on what you’re trying to achieve. If you’ve been lifting for some time, then it’s not surprising to hit a plateau and you might need to shake up your workout. You build based on overworking and the body reacting to damage; if your workout isn’t stressing your muscles then you won’t achieve anything other than maintenance and burning some calories, which in and of itself isn’t enough. I’d ask - are you still a bit sore after workouts?
If you want to get a slimmer look, then 10+ reps with moderate weight for 3 sets is right on. Alternating muscle groups also works here - like if you alternate biceps / triceps neither will get huge but both will get strong and increase your lean muscle mass. Very slowly increase the weight you’re lifting - like no more than 10% per month, but rather increase the repetitions. I am doing this now for rowing, because this not only increases strength and lean muscle mass (and therefore helps you lose weight) but also increase endurance. Also increase the amount of cardio you’re doing to 40-60 minutes 3 times per week; swimming is excellent for this.
If you’re looking to bulk up, do 10 or less reps for 3 sets and do more weight. Don’t alternate muscle groups but alternate days - i.e. M-W-F legs / back / triceps, T-Th arms / chest / biceps, giving your muscle groups time to recover. Week on week, slowly increase the weight so it’s always heavy as your body adapts. Don’t alternate push / pull muscle groups (i.e. biceps / triceps, or hamstrings / quadriceps) on the same day but on opposite days. Limit the amount of aerobic fitness you’re doing as well, as this can result in less muscle mass growth, but still do 20 minutes of cardio 3 times per week.
I’d start using protein shakes like Promax with creatine, and I’d do 5-6 small meals per day with 1 or 2 of them being protein shakes. Also, be sure to get some protein in your body within 45 minutes of finishing your workout. Whey protein works best.
All together that should work to increase muscle mass, and either bulk up (few reps) or slim down (many reps).
The Cocky Watchman - I too am a winter gym-goer; I usually try to get in shape for summer outdoor sports like triathlons, adventure racing, and rowing, and the only time I can really bear to see the inside of the gym is when it’s cold and rainy outside. But if you’ve a grassy space to play in, and don’t mind too much looking like a bit of a tit, then you can do isometric exercises that will give you a weights-like workout for pretty much all your major muscle groups without too much trouble. So you don’t have to lose anything muscle-wise during gym-free summers, even if it’s harder to make it grow.