Both guys and girls should bring/have condoms. Always. I carry them in my jacket pocket even when I'm not going out, sex works in mysterious ways sometimes. The other thing that is fantastic to have for both guys and girls is a tube of lube. Some girls I know dry out when you use condoms but still want more, so get some astroglide and leave it on the bedside table.
My advice to girls (as a guy) is buy the mix packs and leave them at your place. That way you can always find the right sized condoms or flavoured or ribbed, its practical and avoids pretty much any problem barring an allergy to latex. Also, bring along a single generic sized one out of the mix pack, latex is incredibly strechy and even if its too small for the guy to enjoy in that case he should have brought the large size or whatever kind he prefers.
Guys experiment and find what you like, then bring them out with you. I always pack 2, one for that night, the other for the next morning.
And last but not least, buy good condoms! The freebies like lifestyles or durex are crap! They feel bad for the girl and the guy gets minimal sensation through the latex. Personally I like ONE condoms, they always feel good for both parties and two packs in I have yet to break a single one.