An internship in your field of study would be the best summer option. Summer classes, if they will permit you to graduate in 4 years instead of 5, are the best second option.
The reasons for recommending summer classes are:
Many students are living in off campus housing where you are paying for a 12 month lease, so you are paying for the summer anyways. A 5th year of living expenses adds up to a lot of money.
If you have to go a 5th year, does your scholarship cover that? If not, then summer classes aren’t really adding any more expense.
Giving up a year of potential work income to get that additional year in is expensive. Starting salaries for engineers are around 60k. That’s a year of income you’ll never get back. (and it could have a potential impact on your Social Security payments depending on when you retire as SS takes your 35 highest earning years to calculate your benefits)
Taking classes at your local community college, IF THEY TRANSFER, is a good idea to get some of the general requirements out of the way (especially if you are living at home and are in the dorms at school, which you wouldn’t be paying for during the summer)