In your usual meals there's veg that can be added with limited faff eg we are red pepper fans so red pepper goes in 90% meals here. Having a chilli, curry etc? We'd add red pepper. Each meal think if there is more veg you can add. Eg a side of sweetcorn with a burger, a carrot in that spaghetti
Our curry's are usually veg based but even ones that are meat (substitute for us) and simple sauce we'd add a courgette, brocoli, peas, red pepper. Spinach like someone mentioned above can go with lots of things.
Eating vegetarian can just be eating good food, it doesn't have to be some sort of penance.
We eat for example roasted veg with pasta, so for us to have sweet potato, butternut squash,red pepper, courgette on its own with pasta.
For us it's really about finding a love of certain veg. For example I think regular brocoli and cauliflower (especially frozen stuff) is limp and dull. Tenderstem brocoli (more expensive though) is amazing, roasting fresh cauliflower (and carrots) was a complete game changer. Theres lots of veg that I thought was bland and a bit mush before but I was boiling it to hell, and finding out how I liked veg prepared (boiled for less, roasted, stirfryed) really changed that
I think having fresh veg is much more appealing but having frozen veg might mean it's much more tempting to throw in a handful of peas for example, or a bit of spinach to more dishes without the worry of it going bad etc.
It sounds really silly but just find the veg that you like, and use them to their full potential, adding some sad string beans to the side of a meal can just reinforce the "veg is dull" narrative