I've found there are 3 main ways to train during the off season.
For physical training, go for runs. Preferably a route that leads past a school or playground. Every time you pass the playground 10 chin ups hands over, 10 chin ups hands under, 50 pushups etc. Or whatever you deem appropriate focusing on aerobic exercise.
For defensive training, Grab a friend or a mirror and your gun and try peaking out of cover exposing yourself as little as possible. It's good practice to try different objects to learn what bits of you stick out Ideally only part of your gun, and a sliver of your head should be showing at any given time. It looks a bit silly but this is one of the first drills we ever teach anyone who comes to us looking for help.
For offensive training, you really need somewhere to shoot. A basement with a concrete wall or you can hang old carpets up it doesn't need to be long range 30 ft to the wall is more then enough. Then set up some targets. I usually use paper to train for accuracy and pop cans make excellent (free) reactive targets. My favorite is to hang them from the ceiling with a sufficiently long piece of string. When you hit them, they start swinging and you now have moving targets. Don't just stand and shoot them, oh no. Use all the different techniques you've practices from defensive training. Practice snap shooting, under, over, weak side, supine, urban prone everything and switching in between shots or 2 shot or whatever, pushups in between. My favorite barricade is an old chair that can usually be had for free just check your local dumpster.
As always, dress like you fight. Don't switch your eye pro just because your shooting inside. Keep your kit consistent. If you play with a face mask, train with the mask on. If you carry a pistol in game, carry it in practice even if your only doing rifle work that day. If you carry a back pack in game, Guess what your carrying on your run? If your vest and rig weigh 20 pounds, looks like your carrying 20 pounds of water bottles in that bag. Be as accurate as you can be without people thinking your about to shoot up a school.