Caroline, you're doing fine. :-) At this point just let him be active (if he's like my ds, he's doing plenty of that!). He learns from just about everything he does. Let him develop his gross motor skills, fine motor skills, and learn cause & effect (even helping Charlie turn the lights on and off is a good 'game' to play). Also, don't make everything too convenient for him... let him struggle a bit to get to his toys. It'll help him develop his muscles for sitting and crawling. :-)
My ds doesn't like to be on his own for too long, either. It's frustrating because he wants me to hold him but it's because I walk him around the house so he gets to see so much.
He LOVES the mirror now (he could care less when he was 3 mo, which was supposedly when he was supposed to start liking it). :? He enjoys his exersaucer (though he gets tired pretty quickly... or maybe I put him in too late during his awake time). He likes the bouncy chair except that now he's trying to get out. He's really active now... likes to 'climb' things (with my help, of course) and he wants to move on his own so badly!
I didn't get the bumbo seat because I thought it was too much money to shell out for this thing. I forgot to check out ebay, though. (I just looked into it now and I still think $25+shipping is too much.)