Keeping training interesting is crucial for your dog and for you. Nobody wants to train if it turns into drudge work.
Rotate skills that you're working on. Keep varying up what you want your dog to do so he really has to think about what you’re asking him to do.
Remember to work in some fun options too like tricks training. You could take a tricks class in person or online. You could watch YouTube tutorials on some tricks.
I like Zak George's positive approach to training. He has an entire playlist on YouTube called How to Train Dog Tricks (Easiest to Most Difficult).
If your dog isn't into tricks explore agility, nose work, barn hunts, or any of the other many dog sports out there. With each one, you'll be teaching your dog new cues and expecting them to follow your directions.
Rotate rewards too. Vary up the types of food rewards you're paying your dog with, but also work in toys, praise, and pets as appropriate.
If you want to watch your budget for buying treats, I recommend using the autoship option with Chewy.com. I'll swap out which treats I buy each month. Even though my order varies from month-to-month, I still get their discount. Check Pinterest for ideas and inspiration for dog treats and recipes.
Working training into your every day routine means your dog will know training can happen at any time.