Each offset program is problematic in its own way. We can't (feasibly) put carbon back; we can only try to store away what's already out there, or give more people more options to develop without hydrocarbons.
Consider giving to organizations that protect forests and other natural areas to preserve existing trees through policy action or by buying land, especially in places like the Amazon.
If you don't like reforestation, there are companies that take offset money and use it to set up small-scale windmill/hydro/methane systems in developing countries. The goal is to ensure people have access to power without using hydrocarbons. Of course, you have to be sure the governance is good and that the money is going where you think it is.
There's no such thing as a true "offset;" any action can try and reduce future emissions, via saving a tree or offering someone renewable power, but you can't really take back your carbon.