Rubber trees. Tree for paper is also an option. It takes seven years before you can cut rubber; you might need to co-work with some experienced cutters doing the work; the share is often 40/60 or 50/50. Wood for paper, the trees can be financed by the paper manufacturer.
Prices for land can be very different – you, cannot own land as foreigner, so it needs to be a Thai spouse or partner, or through a Thai company limited – and also depends of how good the land is for other purposes, like growing rice. When I bought land the price was around 10k-20k baht for low grade and low title deed rice land; and 40k baht for nor sor 4 (Chanote) title prime rice land per rai (1,600 square meters). Today the same kind of land is traded for around 70k baht and 100k baht per rai.
Be aware of title deeds. Some are kind of ownership – for example nor sor 3 variants – and can be upgraded to full ownership (i.e., nor sor 4). Other title deeds can be agricultural land given to be held by families and not sold – even it's widely traded – these titles are not registered by a land office. However, at the moment the agricultural land titles can begin to be upgraded to nor sor 4 (Chanote) title.
As an investment, you shall calculate it well, before you do it. Agricultural land gives a low average percentage in outcome (or dividend) of the investment. The gain is the expected increase in land value when it once get sold.