Birthday banners can be made or bought. We did both and scattered them around the house, both inside and out.
The preschooler and I cut strips of colored paper with the shaped scissors and attached a balloon. I got my husband to help with the ladder to stick them to the ceiling every 3ft or so, it looked much nicer than appears in the below shot. For some reason my high/ceiling shots don't come out as well (maybe no focal point?).