Most fish is very easy to make so long as you season it and don't overcook whatever you've got. Salt, pepper, old bay or garlic and herb and baking is really all there is to it.
A couple sides that are really easy to make for pretty cheap:
>Any roasted vegetable
Preferably something green, like green beans, broccoli, okra, or Brussels sprouts (these take a little longer). Wash, dry, and toss on a well-oiled baking pan, coating evenly with more oil, salt, pepper, and any spices you want. Bake at 375 F until the lovely smell of roast veggies fills your house, or if you're like me until they start crisping for a crunchy texture. In that case you'll want to flip them around halfway through cooking. Most veggies only take about 15-20 minutes.
>Cheap garlic bread
Purchase a cheap baguette or loaf of Italian bread at your local grocer. Slice lengthwise in half, or into little hockey puck sizes. Coat the top with either a thin spread of butter or drizzle with olive oil, sprinkle with salt, pepper, parsley, and garlic powder. Bake in the oven for about 10 minutes at 400 F and optionally turn on the broiler for a minute near the end to give the top a nice bit of darker color.