Cardi B has at least one bright idea about what she can give Offset for his 30th birthday -- and she's confident it will top his present to her ...

Anyway, we got to talking birthdays, because her husbands is coming up in a couple months -- and he just hooked her up with a new house in the Dominican Republic for her born day.

Cardi might not have to get Offset a house or anything material at all, because she DID just deliver their son, who hit the big 5-0 himself on Sunday.