I know that as College Students vacations seem all too short. But realise that you will NEVER have so much vacation time again in your life. Don't just let them happen... plan them well.
1) You may well need to get a job. If you get the chance try to get a job where you think you will have the most evangelistic opportunities. As an student I worked as a sales assistant, a barman, a statistician, and an office junior. (I had 6 years as a student) All of these were good environments for good conversations with colleagues and/or clients.
2) Your time as an undergraduate may well be your best ever chance to go on a slightly longer short term missions project. If you possibly can, I would suggest that it is a great idea to dedicate a substantial part of at least one of your vacations to an opportunity where you will be able to spend time with missionaries on the ground in a place where the gospel is little known. I had the privilege to spend a month in North Africa as an undergrad. This opened my eyes to the Muslim world, the life of missionaries and the power of the gospel in ways that have certainly shaped me and my ministry even 15 years later.
3) Read good books. When you are away from your studies, read good books.
4) Read the whole bible. vacations can be a time when devotions lapse due to a lack of routine. They are actually the best time for getting substantial bible reading done. It takes about 70 hours to read the entire bible. Block out a reading plan to get through in a vacation. Make notes, summarising the message of each book. A loose leaf bible (On special offer here) is a great resource for making notes.
5) Pray for quality time with old friends. It's easy to drift back into old patterns of behaviour with old friends. The Lord may have changed you dramatically in a semester. Do let that change be invisible to your friends and family. Talk openly about what the Lord has been doing in your life, both with Christians and non-Christians. It is as we let our light shine before men that they will praise our Father in heaven. (Matt 5:16)
6) Note that all this takes planning. Start planning the use of your vacations 6 months in advance.