1868-1870: The War on British Johor
In 1868, the Kingdom of the Netherlands asked new king Rama V to attack British Johor, which Rama V accepted and began secretly mobilizing. By 1869, the Netherlands were urging all of Siam to attack Johor, which they started doing. The Netherlands later started supporting them with arms, and British Johor was defeated by 1870 (although no mod event was made OTL, I checked with Wrto12).
1873-1875: France, Laos, (and a owner in OTL)
Siam faded back to recover from fighting British Johor, until in 1873 France made a deal to buy Laos, which Rama V declined at first. (On April 2016 in OTL, Yellowcat5 decided to own Siam in ATL 1874.) France once again made an offer of protection and trade for Laos in return, and this time Rama V accepted the deal. Afterwards in 1875, France sent 10,000 troops to occupy and establish basic roads in now French Laos.
1875-Current: Putting Siam on the Map
Later on in 1875, the Kingdom of Afghanistan offered to make an alliance with Siam, and Rama V accepted to give Siam its second ally. In 1876, Siam decided to create a Hawaiian embassy in Honolulu, and in return was offered into the Honolulu Pact from Bora Bora (and later accepted towards the start of the 1877 Suva Conference). Bora Bora later offered a six month trade agreement with Siam in 1877, which was accepted by Rama V and has since run out. Most recently, Siam was one of the countries that received a huge amount of weapons from South Germany (still in 1877), giving up some natural resources in return.