(This is money I'd otherwise spend on consumption, e.g. more flights. I'm talking about consumer-level money, so on the order of hundreds to thousands USD.)
The first caveat is that the trees are planted, but may not necessarily live, and even if they do live for a while, it may take decades before they actually cause an "offset".
The second is that trees that are planted are sometimes harvested, causing the offset to be negative, given that fossil-fuels are almost certainly used in the harvesting and processing of the wood.
The third is that the trees are likely to be plantation trees -- ie, a monoculture with little to no wild-life value. Wild-life value is important, because trees aren't things, they're normally part of an ecosystem that sustains itself, adds to soil carbon, and so on.
Or you could find a way to buy and plant and care for your own trees, and use your own labour, which would be a thoroughly honest way to go. Having planted a lot of trees, there's nothing like looking at a tree you planted 10 years ago and is used by birds and insects and shade for animals.
Lastly, if you do have thousands of USD, possibly the best carbon offset is to either fund a activist group to get in people's faces about the larger problems, or to buy political influence to pass more environmental protection legislation.
With that said: I do realise that some locations are hard to reach without air travel.
It can be quite tricky to find serious organisations to donate to for the purpose you mentioned.
One organisation that I believe is serious is Naturarvet. The name translates roughly to “The Legacy of Nature”. Their mission is to protect old forests in Sweden. In Sweden we have lots of very young forests. But we don’t actually have much old forests, and this organisation is trying to change that. Nearly all forests in Sweden are used for making lumber or pulp. So Naturarvet don’t actually plant any trees, but instead try to preserve forests to let them grow old. As 'MrVandemar pointed out, planting trees is only part of the job, then we have to refrain from chopping them down.
If you’re looking for something in the US, Sierra Club Foundation might be a good place to donate to. They have a score of 98% on Charity Navigator.
There might be similar organisations in other locales, but these are the I know of. You can find more here: https://www.charitynavigator.org/discover-charities/best-cha...
Hope this helps! Would be interesting to hear your thoughts regarding my suggestions (:
Links, all in English:
https://naturarvet.se/en/bevara-skog/skogsrutor-och-paraplyt...
https://naturarvet.se/en/bevara-skog/bidra/ge-en-gava-pa-90-...
[1] Assuming they make sense where you live.