Student evaluations can be informative, but generally only help the next semester's students. Since I started teaching in 1999, I've used the "week 4" evaluation. I've used index cards and online survey tools like SurveyMonkey.
For index cards, I ask two questions. Students do not write their names on the cards. On one side, they report about something they've liked so far about class. On the other side, I ask them to suggest one thing they would like changed. I tease that I can't change things like our scheduled day to meet.
For the online survey, I typically program a whopping three questions: the same two from above and "Is there anything else you would like to add?" If I have access to a learning management system, I link it from there and ask students to log-in during class. If we're not in a computer lab, I use the announcement feature to force out the survey invitation. In an accelerated summer semester, you might employ this tactic at the end of week 2.
Brace yourselves for the results, but generally students like that you're even asking. I look for themes and try to make changes where possible. I also tell students that I'm making the changes based on their feedback. Who doesn't like to think they've made a difference? I have found that I can always implement something they have suggested.