Lass uns ins sonnige Griechenland fahren!
What excactly is "ins" here? In + das ? We have Griechenland as feminine. Danke!
In addition to what @Sonyfreak already said: compound nouns always derive their Geschlecht from the last word they consist of. For example:
die Stadt (fem.)
das Stadtparlament (neut., "das Parlament")
der Stadtparlamentstagungsraum (masc., "der Raum")
Addendum: as correctly stated in @Uwe's comment, there are a few exceptions to that rule (as with any grammatical rule - as I have tried to explain here), especially names of proper places. I.e. "Frankfurt" is neuter despite "Furt" (fording) being feminine.
"Ins" is a compound of the two words "in" and "das". Griechenland is neuter, because it derives its article from "das Land".
I think part of the confusion here is the inflection of sonnige, since the -e ending might seem to imply that the following noun is feminine. The -e ending is correct though. There is a das, even though it's reduced to part of ins, so that means you use the partial (aka weak) inflection. The rules for partial inflection are to use -en with plurals, genitive and dative, and with masculine in the accusative case, and to use -e otherwise. This is neuter in the accusative case (since Griechenland is the destination of travel) so it's the "otherwise" -e. Note that a dative Griechenland would be preceded by im rather than ins.