+1 to delay and reverb being big effects for ambience.
I'll also add a couple suggestions: You might try a Boss PS-5, I've got one and it can do some neat tricks. I've also heard some super trippy stuff from the Boss PS-3. They're on the PS-6 now, so you'll have to find both of these on the used market. I suspect the PS-3 will be out of the $50 range, but it might be worth picking up regardless.
The second would be to see if you can find a decent used multi-effect. Since you won't be using it very much, a multi might have more to play with for the same price... reverb, delay, flanger, phaser, chorus, vibrato, maybe a ring modulator, maybe a drone (sitar) effect like my old Korg AX1B... all sorts of stuff to play around with.
Also, I'm not a fan of EHX pedals at all (topic for another discussion
), but one of my favorite sources for ambient sound ideas
(although I don't really do a lot in practice) are the EHX Effectology
videos. Man, the guy that does those videos is an effects savant... I can't even wrap my brain around some of that stuff. lol Maybe you can find some inspiration there too?