Some good categories to start with could be home and kitchen, kitchen and dining, sports and outdoor, and video games. Those categories shouldn't require ungating but you may come across brands within these categories that require ungating.
Another good way to start might be picking a category or niche industry you having an interest in and looking for US-based suppliers in that industry.
In the end, the best categories you work in are going to be the categories you find the best supplier in. We've seen students do great in toys and games, video games, clothing, pet products, etc. There is no limit on what is a good category, it all comes down to finding a good supplier within that category.