My son just turned 6 months old today...so I still remember 5 months quite well.
He likes his play mat/activity gym (Tiny Love). We change the position of the hanging toys.
We have these small stuffed animals with bells in them... very popular...as he can hold them.
Teething rings/beads are a toy and useful too. Especially the kind that can go in the 'fridge.
Tupperware containers are fun to play with.
We had this thing called "Hop 'n Pop" which he loved by Fisher Price. Unfortunately it's at Grandma's house.
He loves to swing... so we go to the park and hold him on our lap and swing gently.
He loves to be held and bounced on an exercise ball.
He loves to sway back and forth. Usually I'll face him towards me and sing "Row Row Row Your Boat" or some other silly song.
Knee Rides... I usually hum the "Charge of the LIght Brigade"
Reading...board books (Sandra Boynton books)...which are really teething books
Airplane rides... lie on my back, put him on my knees...and move him forward saying "Zzoooooommmmm"
Hope that helps!