Well, for a start they can repeal Roe vs Wade thus making abortion completely illegal in the US
they might manage that with just a 5-4 majority but it’s a pretty close call and I’m not sure how partisan the current Justices are or how loyal to Trump or what their exact religious/social beliefs are
but if he/Pence get a 6-3 or 7-2 majority then the sky is the limit (given that the Republicans have both houses) in terms of what they might be able to accomplish - amendments to the constitution, high legal precedents, reverse State & Circuit court decisions etc. Basically any legal challenges to any Trump Executive Orders or legislation will be nigh on impossible to win/get bogged down for years