- Right off the bat, ask them point blank if there's anything specific they want. You can kind of "pressure" them into it by giving them specific requests of your own.
- Something that reminds them of you or lets them show you off is nice, especially if you don't spend a lot of time together. One year I got my parents a custom-printed mug with a picture of myself on it. It was specifically requested, but your parents may like this either way. If you go to ___ University, then you can buy them a mug or sweater that says "proud ___ University mom" or "Someone I love goes to ___ University", or something of that sort.
- Something that you've acquired in your independent travels/adventures, that you probably couldn't get otherwise. Once you start travelling without them, whether for work or vacation, it's always nice to get a souvenir to show that you're still thinking of them. Stay away from tourist-y stuff if you can help it. Non-perishable consumables – sauces, jams, preserves, coffee beans – are great if travel restrictions allow for them, but locally-made crafts can work too, assuming your parents have the room. Candles are kind of perfect for this, since they don't occupy permanent shelf space, but you can avoid the TSA breathing down your neck.