As far as I know, the structure "I thought we could/would do....." can be used for 2 purposes:
1-When you found that something was unexpectedly different from what you thought it was. For example: I thought the dishwasher was broken, did you get it fixed? (unexpected situation)
2-When you make suggestions: For instance: I thought we could go to the lake this weekend. (making a suggestion)
Now, If others didn't come to the lake and it was just you who came, the plan did not take place. So, when you see them next week, you want to show your curiosity about it.
Can you use the same structure (I thought we could go the lake this weekend) to refer to your curiosity about why the suggested plan didn't take place.? (Just like you used the same structure to refer to your curiosity about the diswasher not being broken.)
Or should I say "I thought we could HAVE GONE to the lake the last weekend!"? to show that the plan did not take place?