1)
a. Did you buy anything intesting at the weekend? OK (I know you went shopping)
b. Did you do anything interesting at the weekend? OK ( I know nothing about your weekends)
Did you make anything interesting would only be used if the speaker knew you usually did some kind of craft/ cooking etc.
2)
a. Did you have a beach holiday? This is OK, if you knew the person had been on a holiday, but you weren �t sure where.
A more common use would be �We had a beach holiday this year, but we usually go to the mountains.
b.Could you.... Not correct unless you knew that there was some problem with their ability to get to the beach.
Could you go to the beach on your holiday or was the road still closed?
Did you go to the beach on your holiday? (this is the usual way to ask)
c. Was your holiday coastal? (This sounds very odd) I would say Did you go to the coast?