70's to 80's is about right for early April. The humidity usually hasn't started yet, so it's very nice out.. probably the best time of year, it gets hotter than hell starting in mid-May. Houston is a very big place geographically, so you should stay near where you're going. That said, avoid anything near Hobby airport or east of downtown. Also avoid the north side of town (Greenspoint area), unless you're going all the way up to the Woodlands which is very nice. If you're staying inside the loop, Downtown and the Galleria are the touristy/entertainment areas, and the Texas Medical Center area is also okay, as there's a train from there to downtown. The far west side (Katy freeway outside of Beltway 8, Westchase, and Sugarland) is upscale residential with lots of hotels, but avoid the hotels on 59 between Downtown and Sugarland.
Hope this helps.