I once divised a Levelling up plan in my head, where if you level up 5 levels before each little bit ie

5 levels before going into the reactor
5 levels after the train (by going into the train graveyard)
5 Levels before going into the 2nd reactor
5 Before fighting Air Buster
5 Before getting to Aries Hous
5 Before getting to the Wall Market
5 Before getting back to Sector 7
5 on the way up the Pillar
5 on the way to the Shinra HQ
5 on floor 60
5 on the floor with the item coupons
5 on the floor with the Midgar parts
5 after being in the cells

and that takes you to outside midgar at Level 70

you could easilly make level 99 before the end of disc 1 and its not as time consuming as other methods. except i get bored too easilly and go up like 2 levels and can't be arsed lol