Trinidad has the hottest peppers in the world. Ask for some Trinidad Scorpions and Seven Pods. Get all the colors: red, yellow and brown. There's probably more. Get those too. You can smuggle the seeds back.

There is nothing much in Trinidad that you cannot find in the US. Only if you are looking for a specific cultivar of something from Trinidad might it be worthwhile.

If it is not prohibited, a phyto from Trinidad will allow you to bring in up to 12 articles I think without the need for a permit.

You cannot bring back anything mango related easily.

I just remembered a place that might have some rare trees. It's called La Vega Estate. It sometimes has grafted rambutan, durian, pulasan etc for about $10 usd. Bare rooted and with a phyto which you can get for free at customs you should be able to get them into the US if they aren't prohibited.. Everytime I've contacted La Vega they have been out of stock of the trees I wanted but your uncle might have better luck.

One problem with getting the phytos though is that sometimes they say you need a US permit for them to give you a phyto. You should not need the US permit but their insistence reinforces the view that customs agents around the world are equally as incompetent.

Here is a link that might be useful: La Vega

If he get seeds of anything, it's best to just mail them back in a bubble envelope. And like Murahilin said, La Vega is one of the better nurseries. If your uncle visits one of the markets on a Saturday, he may get some interesting fruits (for the seeds) depending on what is in season. Also, there is a nursery in Chaguanas that sometimes has some interesting fruit trees. Just have him ask anyone where the big nursery in downtown Chaguanas is. They'll usually tell him "dong the street and 'arung' the corna" (lol)
Also, be prepared to get a hard time at the airport here. Even with a phyto certificate, you'll have a rough time.