What are some colleges that are preppy and conservative but still have great social scenes and greek life?
For a snapshot of an earlier time, these colleges were listed under The College of Your Choice in the Preppy Handbook:
The Top Ten
Princeton
Hamilton
U of Virginia
St. Lawrence
Babson
Hampden-Sydney
Lake Forest
Sweet Briar
Hollins
Pine Manor
The Runners Up
Amherst
Reed
Williams
Georgetown
Colorado College
Trinity (CT)
Colby
Vassar
Connecticut College
Wheaton (MA)
At this time, however, many of the above schools take pride in their diverse student bodies, as even some of them may have when the PH was published.
In terms of your personal criteria, you may want to consider Washington & Lee, as well as a few of the above.
Washington and Lee University is stereotyped as preppy. It is also more conservative than other top LACs. Greek life is a part of the social scene. The University is less ethnically diverse than other LACs, although I am not sure if that will be a deal breaker for you.
The Preppy Handbook is completely out of date. I can’t believe Reed ever was preppy, but it sure isn’t preppy now.
These schools would be literally preppy:
Colleges by Concentration of Representation for One Northeastern Private School, Forbes’ #1 Ranked Trinity School (minimum 5 TS matriculants, 2011-2015)
- Hamilton
- Harvard
- Colgate
- Columbia
- Brown
- Yale
- Kenyon
- Amherst
- Dartmouth
- Penn
- Wesleyan
- Bowdoin
- Chicago
- Pomona
- Middlebury
- Williams
- Duke
- Princeton
- Emory
- Cornell
Agree that the Preppy Handbook is out of date and not worth bringing up to reference the OP’s question.
My opinion on some preppy, fairly conservative colleges:
University of Virginia
Washington & Lee
Several Catholic colleges like Holy Cross, Providence College, Boston College to a certain degree
Sewanee
Trinity College (CT)
Southern Methodist University
Wow. Vassar is and never was preppy.
Maybe Dartmouth and Union in NY state.
There are also different kinds of preppy. There is a certain kind of New England preppy that is more crunchy, hippie preppy. Think Grateful Dead and grandpa’s old LL Bean Field coat. In the days of the old Preppy Handbook, some of those types would possibly have attended the likes of Vassar, Colorado College, Reed, and still do. But, as they tend not to be conservative, I would guess that is not the type of preppy the OP is looking for. My guess is he/she is referencing a more conservative set wearing Vineyard Vines, Southern Tide, and Chubbies which will be found in quantities at the schools I listed in post #5.
Providence, Fairfield, Hobart &William Smith, Babson, Bentley, Fordham, Trinity CT…
Adding Bucknell and Vandy
Not at Vassar. It was not a preppy school and still isn’t.
Percentage who came from private/prep school is not at all the same as “preppy.” Preppy is a stereotype of a certain social environment. For example, the University of Chicago and MIT get tons of students from prep schools, but they sure ain’t preppy.
Post 5 is a good start. I would add Middlebury, Dartmouth, Vanderbilt, Georgetown, Miami of Ohio, Notre Dame, Davidson, Duke, Colgate, Emory - and for the California variant - USC, Claremont McKenna, and Pepperdine.
“preppy” used to mean “coming from New England prep schools”, in that sense Vassar would have had many students coming from girls’ boarding schools.
Even the “preppy” handbook has a different meaning than just that, and today it really doesn’t mean “Putney” 
And Duke, Gettysburg, and Denison (although the latter is working hard to shed this reputation. ) This kind of environment exists at several levels of academic intensity and all over the country, so it could be helpful to know what level of selectivity and what part of the country interests you.
And yes, @doschicos’ description of the true NE prep of the 70s/80s rings true. Maybe with that blue sweater with the white flecks from LL Bean…
Vanderbilt, Washington & Lee, Colgate, Connecticut College, and UVA come to mind
And . . . WAR IS PEACE, FREEDOM IS SLAVERY and IGNORANCE IS STRENGTH.
I had one of those sweaters, @gardenstategal. I’ve wondered many times what happened to it and wish I had kept it.
They still sell them @doschicos ! I got rid of mine only 5 years ago, but still have the (now) treadless boots and a box of bootlegged Dead cassettes…
I still have the boots, too. They need repairs but so do I after so many years. 
I was recently cleaning out some boxes and came across old prep school yearbooks. The fashions and styles on the guys have held up surprisingly well. Less so for the girls.