I interned at an excellent startup last year and they gave me a return offer for next summer. I love working there but would like to branch out and work at other places to get an idea of different work cultures. What's the best way to tell them I would like to work with them in the future? I am concerned that I am burning a bridge.
Something like "I believe gaining experience in different environments will be a good way to start off my career. I would definitely love to work with you in the future and bring my experience and impressions from the other companies I worked for on board." There is a chance they might perceive it as a soft rejection but most likely they will not see it as you burning a bridge.
If you do come back to the company a few years later with the experience to show for it like you told them and they have a need for the skills you would bring on board there is no real reason they would see it in a negative light.