A lot of the LAC’s that have engineering tend to have a smaller selection of engineering majors, so something more specialized like Environmental will be harder to find in that setting. Even harder to find a school that has both engineering stuff and design stuff, AND the ability to combine them. (The bigger the school gets, the more separate the “silos” tend to get.)
I agree with those above who suggest UVM - it’s small enough to have a relatively LAC-like feel, and to make cross-disciplinary stuff possible, and it has a very strong environmental bent. The Enviro Engineering program there might be a great choice.
A more specialized small school possibility would be SUNY ESF, which is adjacent to Syracuse University and allows cross-registration in SU courses (for an affordable SUNY price - even OOS is relatively inexpensive). It has several environmentally-oriented engineering majors as well as some design options i.e. the minor in landscape arch. studies.
Union College has an Enviro Engineering minor, but not much design stuff per se. Clarkson has an environmental engineering major. So do Lehigh and Bucknell. Also Stevens (esp. if you have any interest in a coastal/ocean focus)
CU Boulder does have a really nice environmental design school - small and personalized even though the university is big. They also have enviro engineering… but I don’t get the sense that there’s much cross-talk between EVND and engineering, and it would be hard to find a liberal arts feel.
The enviro engineering program at CO School of Mines might come close to what you’re looking for, though. (Definitely not liberal artsy, but small and close-knit, with lots of nature!) https://cee.mines.edu/bs-environmental-engineering/
Harvey Mudd (STEM-oriented LAC in the Claremont Consortium) has an Environmental Analysis concentration that is available to engineering majors, and their engineering curriculum is very design-oriented. (Not necessarily an easier admit than Tufts, though.)
Lastly (in the not-easy-to-get-into category), Olin has a very design-oriented engineering program, as well as this certificate program http://bow3colleges.org/sustainability-certificate