1. Commit at least 2 days/week to get better!
Whether you're in-season or not, training at least 2 days per week is optimal for long term development. If you're in-season or just learning to train, then 1 day per week is acceptable, though I would still aim to train twice per week!
2. If you're not in-season, train more than you practice your sport
Unless the summer is your main competitive sport, your biggest bang for your buck is going to be improving your athletic abilities this summer. You've probably just spent the last 8 months on the ice, the court, or the field. Any physical improvements you make are limited practicing your sport. In order to come back as a better athlete this next season, spend more time training!
3. Do a proper stretch and warmup before each workout
Take at least 10 minutes to stretch and do a dynamic warmup before you train or work on your individual skills. Warmups are shown to increase mobility, prepare the body for training, and help reduce the long term risk of getting injured.
4. Sprint
Sprinting is one of the best and specific ways to improve speed! Even if your sport doesn't involve running, it is a great way to train explosiveness as well. Even for hockey players, improving sprint time off-ice is shown to improve on-ice speed as well!
5. Jump
You don't need to be a volleyball or basketball player to do jump training. Jump training is a great way to improve lower body explosiveness and power. Focusing on proper landing mechanics can help reduce the chance of lower body injuries as well!
6. Do some sort of resistance training
Even if you don't have access to a gym, there are ways to get stronger this summer. Exercises like pushups, single leg squats, pull ups, single leg deadlifts can be done with inexpensive tools you can find at a fitness store (like medicine balls, light weights).
7. Work on the small muscles too! (core & other stabilizers)
It's always fun to work on the flashy muscles. In order to keep your body balanced, everyone should work on core muscles, glutes, and shoulders stabilizers to keep the body working symmetrically, and limit the chance of imbalances. Most of these exercises can be done with body weight, bands, or low weight dumbbells.
8. Do your conditioning!
Nobody likes doing cardio, so if you're able to do cardio twice per week, you'll separate yourself from the competition. Being strong and explosive is only as useful as your ability to recover from it and do it over and over again. The best type of conditioning is high intensity intervals since it is the most efficient way in terms of time spent, and improvement compared to steady state cardio.
9. Pre-fuel and re-fuel!
Whether it is getting the most out of your workout, recovering from your workout, fuel is one of the keys to performance. Having a snack before your workout will help you get the most out of it. After your workout, you should have a recovery strategy as well to refuel. Something like a protein shake should be a routine every athlete should adopt. Protein drinks aren't necessarily superior to real foods, they're just easier to be more consistent with since you don't have to worry about spoilage. Just mix and drink!
10. Be consistent and embrace the basics
Everything on this list is easy to do. It's also easy not to do. The basics can also get boring to do. The difference between good and great athletes, are that great athletes embrace the daily grind of working hard and brilliant at the basics! The average person will follow this list for a week or two, then fall off. If you do this all summer, you'll make big improvements over your competition!