How do you go to Gensokyo? Why, there are lots of ways to go to Gensokyo. Pick any of the following methods as you please

65. Try eating 9 of those 99% cacao chocolate ice creams all at once. An angry Cirno will smash through the Hakurei border and make a interdimensional gap near you. Make sure you jump through the gap as quickly as possible, otherwise i) the gap will close, and ii) you'll be reduced to nothing but a bunch of ice cubes on the floor if you don't jump into the gap while it's open.
66. Fire an onbashira into the air with some powerful cannon while you're standing on it. Make sure you don't fall off of the onbashira! You'll be able to land in Gensokyo.
67. Skydive from a plane without any parachute. Throw some fried tofu around in the air and pray for help to Yukari. If you're lucky, she'll gap you into Gensokyo!
68. Scream out "I do believe in fairies! I do! I do!" in all Shinto Shrines.
69. Hijack a subway train and reach the speed of light. Lol Yukari.
70. Play human Touhou on a bicycle in a crowded street and graze 9999 people. Bonus points for stealing point items from them like Marisa.
71. Feign an automobile malfunctioning and call 1-800-555-KAPPA. This part needs some patience because 99.9% of all the time, you'll get a response that the line is busy at the moment. Upon managing to call the phone operator, make sure to describe your automobile malfunctioning in the clearest and most brief way possible because you'll be charged $103 per second of the call. Needless to say, your call will be monitored through recording your voice and, to some extent, videotaping your face! After they accept your distress call, an armored Kappa will arrive on Nitori's custom-made vacuum machine. As soon as the kappa becomes busy with your car, ride on the vacuum machine and FLY THE F**K AWAY FROM THE KAPPA because as soon as she realizes her vacuum machine has been stolen, she'll fire all sorts of heavy weaponry at you! You may create a warp gate into Gensokyo by turning on the vacuum and aiming at any spot in the sky for 2106 seconds. Remember that the warp gate will not form if you aim away from the spot for even 1 nanoseconds!
(WARNING: The YMPD (Youkai Mountain PD) Will have a warrant out for you by the time you arrive in Gensokyo. Lay down low for a while until the dust settles and the inhabitants grow less suspicious of you)
I gave you four ways that make total sense. Hell, this thread is FILLED with ways to get into Gensokyo. It's like a Lonely Planet: Gensokyo up in here.
Speaking of which, why don't we also make some suggestions for sightseeing and eateries for those who are new to Gensokyo?