I'm making fish tacos, what are some great side dishes?

Not exactly a side dish. But I love Pico de gallo on fish tacos. Here's a no-recipe version. http://www.thekitchn.com/how-to-make-pico-de-gallo-salsa-fresca-cooking-lessons-from-the-kitchn-25757

A Mexican style fruit salad. What ever season fruit is available. But mostly it's watermelon cubes, mango cubes, orange slices, apple. (orange and Jicama are a big element in this...and you can play around the other season fruits)
That gets an addition of 'non fruit' type things: Jicama (cut into match sticks), Tomatoes, chili peppers diced, Cucumbers, Cilantro (steams chopped fine).

Then it gets a dressing of lime and olive oil. And a good dose of crunchy salt right before serving.

Rice and beans

I love fish tacos of all kinds. Depending on the ethnic bent, the sides are very different. Mexican fish tacos love to be paired with avocados, rice and beans and radishes. Asian influenced fish tacos are wonderful with crunchy salads with peanut based dressings or rice vinegar flavored slaws.

For the recipes that don't require the corn to be on the cob - frozen corn is actually pretty good!

Depends when you're planning this - and where- it will be a while in most places till corn arrives!

Yikes, another one:
http://food52.com/recipes/17913-charred-corn-and-avocado-salad-with-lime-chili-and-tomato

You inspired me to make fish tacos this weekend and I loved the corn ideas. These all sound great too:

Traditional Mexican street fair corn:
http://food52.com/recipes/12330-mexican-street-fair-corn

Dilled Crunchy Sweet Corn Salad:
http://food52.com/recipes/12928-dilled-crunchy-sweet-corn-salad-with-buttermilk-dressing

Roasted corn with lime, parmesan and chili
http://food52.com/blog/1101-roasted-corn-with-lime-parmesan-and-chili

Corn salad with cilantro and caramelized onions
http://food52.com/recipes/13784-corn-salad-with-cilantro-caramelized-onions

I make this with fish tacos all the time, and also do rice and beans on the side.

http://food52.com/recipes/12818-cabbage-vinaigrette

http://culinaryarts.about.com/od/salads/r/watermelonsalad.htm watermelon salad

Wilted vinegar based cole slaw.

Sweet potato fries (baked or fried). Corn on the cob or a corn salad (grilled corn would be good in that).

Also consider a crunchy salad (cabbage, scallions, asparagus, lettuces) with some fresh avocado slices. There are great ones on the site: just do a search for "avocado" and "salad."

Arroz con habichuelas (rice and beans, you can make the beans vegetarian or non-veg with pork), unless you're using rice or beans in the tacos. What ingredients are you using to make the tacos and how much time do you have?