It has been many years since I went to the gym... partly because of exactly what you say - it gets boring. I was a world class athlete in my long-ago youth, and needed to be strong. It isn't necessary to do a traditional weight training routine to get strong. You probably would want to do your regular routine some of the time, but you can mix it up. For instance, we used to do a pullup routine: 10 pullups with hands wide on the bar. 10 with hands shoulder width apart, 10 with hands together. Do all of the above twice, once gripping the bar with palms facing you, once with the palms facing away. That's 60 pullups. Then when you get really strong, we'd grip the bar with one hand (say, the right) and then hold onto your right wrist with your left hand, and do 10 pullups, then switch. Do those with palm facing both ways, too. The entire routine is 100 pullups. Takes a long time to work up to doing that many, but boy do they make you strong. And varying the grip and width of your hands works the muscles from different angles.
We used to ride the bike or run on the treadmill for 5 minuted, then go to the pullup bar and do a set of pullups. Go back and ride or run another 5 minutes, do another set of pullups. Throw in a set of situps or crunches. You can, if you want to, make any weight training routine into both aerobic and non aerobic, by doing quick light sets, and doing them as quickly as you can, and still do them correctly. If you can get to the gym when it is quiet, you can often set the weights on your bars or machines ahead of time, so you can go quickly from one to the next.
Get involved in another sport or activity that builds strength. I became a world class athlete without ever having a gym membership. I ran, I did pushups, situps, crunches, pullups.We did plyometric exercises, like jumping onto and off of a wooden box (be careful that it won't slide under you). We used bleachers. We stood still and jumped as high as possible, we jumped forward as far as possible. We picked a line on the floor and jumped side to side over it, back and forth over it. There are many books available on plyometrics.
I wish there had been Pilates when I was training and competing; some of the strongest people I know do Pilates.
Don't forget to warm up thoroughly, and to cool down, and to stretch after every workout. If you can find a workout partner who will challenge you, and who you can challenge, that really helps, too. Working different muscles every workout is a good idea. You won't gain strength as well if you overwork yourself. You are right, too, that it needs to be fun. If it isn't, it just becomes work. You should look forward to your exercise time. Good luck!
