Baikal is not crawling yet, so I am sure things will change once that milestone is reached. She is also just starting on food, so I do not have much advice on that yet. Here are the items that were useful during this time.
3-6 Months
Infant tub-most people use this right away, but we were a bit late on the bathtime routine. Baikal does not like to lay back for her bath, so I am glad I got one that can support her while she is sitting up.
Exersaucer-I got the cheapest one and it has been fine. I was recommended to start using at 4 months, but she didn't really get interested in it until recently.
Jump and Go-I added this to the list, post-publishing but it was so great that I had to include it. It hangs in the doorway and baby jumps up and down in a bungee-like contraption. Baikal loves it and it keeps her occupied while I do other things. Woohoo!
Big Baskets-I have a few big baskets that I got at WorldMarket. They each have a few toys and a blanket in them. I put them in various rooms around the house so that Baikal has a clean blanket and stuff to play with while I do other things. Once a week I gather the baskets and clean the contents.