it really depends on what you're looking to achieve from suplementary training. anyone who lifts big weights will know that it can have a detrimental effect on your flexibility. if you're going to start weight training, you might want to increase your flexibility training as well, just so that you don't end up like all of the wieght lifters i know who can't touch their toes. my teacher has always emphasised strength and flexibility...you'll strengthen up a bit even just through flexibility training.
cardiovascular training is excellent, most martial artists i know go running or cycling several times a week. you just need to be careful about training outside when its very cold or wet, as its not too good for the lungs.
tai chi is good for developing control over individual muscles - especially the deep ones that some other forms of exercise don't always help, and i can imagine yoga would be good also. shaolin styles of kung fu offer a fairly good workout, as they involve a lot of jumping and quick movements. plus kung fu requires stong legs, and builds good stances - something that not every aikidoka has
but then there is the problem of changing between stances in different styles (kung fu bow stance is so nice and comfy)
i would advise doing something you enjoy, 'cos that way you'll stick at a routine. you could always get together with a group of fellow students, as training with others always makes things more fun.