Young couple from US need place from July to mid-August

My boyfriend Adam, age 22, has an internship at the Max-Planck Institute for Astronomy for the summer. I, age 22, am coming along as well, so we will be living together. We need a furnished place from July 1st to August 15th but the individual dates are flexible. Potentially we could move in at the beginning of June as well. We currently have a place to stay in June, and we can come meet with you in person.

We would prefer a bed big enough for two, but we are flexible. We don't want to stay anywhere that allows indoor smoking.

Location: We would prefer to be close to the Altstadt, and would like to have a short commute to the Max-Planck Institute.

We prefer emails to calls/texts, but we will post a number once we arrive.

Our maximum possible rent is 1000 euro/month, but obviously we are interested in cheaper places as well.