I'm coming to end of my degree in mechanical engineering now I guess (eek) and this is the advice I'd give myself four years ago.
Definitely do a sandwich year. You'll learn more relevant content that year than any of the 3 or 4 you spend at Uni (providing you get a good placement you enjoy).
In your spare time, learn a CAD/CAE package inside out. Don't wait for courseworks requiring you to learn CAD, do it in the first place and you will fly through those modules! My choice was Autodesk Inventor (2010-2013), but others prefer CATIA or ProE (now Creo Elements). I think it comes down to preference really more than anything while you are at Uni.
For group courseworks, if you can pick your group, choose people with the best ability, rather than just your mates. I know it might be difficult but that's a mistake I've made every single year
. It will mean you end up with more work to do, because you are carrying others.
Don't skip too many lectures. It will come back to bite you.
Get involved with Formula student as a fun way to learn/apply what you've learned. I regret not doing this now. My friend did and it's plain to see the knowledge he has gained.
Don't neglect your social life. Monday to Friday work as hard as you can to get that 1:1 or 2:1 you deserve, but give yourself some time off at weekends else you will tire out eventually.
The two main subjects I've found I had to read around most are thermodynamics & fluid mechanics, and solid mechanics & stress analysis. Give theses a bit of extra time.