I've traveled to Wat Umong from Chiang Mai city on both Tuk Tuk and Songthaew, they cost 300B and 70B respectively. I've overpaid for Tuk Tuk since my Tuk Tuk return trip was only 100B.
Wat Umong has a meditation centre where one could stay and meditate. As there aren't many tourists, it is a very conducive for meditation. The place doesn't offer much instruction, it will be best if you have prior meditation experience. White shirt and pants (needed for stay-in meditation) can be bought from the office of the meditation centre.
Besides the Tunnel, pond and museum mentioned by other travelers, there is old temple ruin further into the forest. Do ask the monks for directions.