I'm currently on my 20 minute door bouncer break as my 5 month old (in a couple of days officially, but I want to get used to saying that!

) is happily jumping up and down. This has been a great addition to the 20 minute baby gym break or the 20 minute baby Einstein break hehe. Only major deadline days has caused me to use all three but that's happened only once and never consecutively. Little un's need some time to themselves (as do us mummies) but generally we do a bit during the day.
Playtime is always fun (for both of us, I hope!) We read a couple of books, and sometimes it doesn't matter if the second one is one of Mummies as she's racing to get to the end of the chapter -- I honestly think a baby knows when you're reading to him. That you're sharing something with him.
Some great toys we use for 4-5 mos have so far been, a colourful cloth ball with a tinkling bell on the inside to play catch with, cage rattles, his little monkey textured teether thingy (one of thse clothey noiseless rattles? from lamaze) he's loved his baby rocker chair since about a month old and has started to take to playing with blocks
From 3 months on, I found little K really interested in more physical games - row row row your boat, peekaboo's a classic

having him 'fly' around the flat and then look out the window, this little piggy, round and round the garden etc.. (right now he likes it when I jump alongside with him)
We go to the park a lot as well as shopping malls and more recently museums. We're going to the elements mall today and the history museum sometime this week (more for mummy than him, granted -- but we;ve been taking him out in his buggy since week 1 and he loves looking at things)
Playgroups are great -- we try to go to one regularly but with us living in Yuen Long and the playgroups we like being in Central doing it every week doesnt always work out as too many factors kick in sometimes. He enjoys the toys and babies, and mainly tries to eat well, both.
If all else fails, he has a little stuffed blue monkey we gave him since birth and Cheeky comforts him. And is always there for a good chew