Getting through customs/immigration usually takes only an hour or so but I would recommend a minimum of 2 hours during the week and 3 hours on the weekend. We have been to Aruba many times. A taxi into town is rather inexpensive and is only a 10 minute drive. Consider heading into town and having lunch at Iguana Joes, a cool restaurant, upstairs, with good views. Inexpensive ribs, chicken, etc. You can relax there or walk around to some of the shops.

You could also head to the beach but for an hour or so I don't think it is worthwhile.