1) The only really Dutch word that most closely corresponds to 'hiking' is 'wandelen'. However, 'wandelen' is only used in the sense of 'taking a walk', so that doesn't really match the actual meaning of hiking in English. Therefore, another Dutch has come into use that's based on the English one: hiken.
So that would make:
Laten we gaan hiken! (Let's go hiking)
Wil je gaan hiken vandaag? (Do you want to go hiking today?)
2) Wat denk je hiervan?
3) You can use goedkoop in Dutch in the following sentences:
– Deze schoenen zijn echt goedkoop (These shoes are really cheap)!
– Dat is niet goedkoop (that's not cheap)
However, the other two sentences require a different word:
- Hij is gierig (he is cheap → greedy)
People would understand you I think, if you used goedkoop here but to me it doesn't sound natural.
- Deze schoenen zijn in de uitverkoop (on sale = in de uitverkoop, which directly translates to 'in the outsale').
4) I actually had to think about this one for a bit, but I can't think of a direct equivalent right now, at least not in the context of bumping into someone who you haven't seen for a long time. There's “nog nieuws?” (any news?) but that's more in the context of two people anticipating some kind of message. I would say it's just “Hoe is het?/Hoe gaat het met jou?” accompanied by sentences such as “Tijd niet gezien!” (long time no see), “Vertel me alles!” (tell me everything) etc.
I hope these are sufficient answers to your questions! :)