It's hard to know what to do isn't it? I'm sure she's not bored with you though - you're her whole world!!
I'd also suggest a play mat but try to borow one or find one second hand. Only because she'll "grow out" of it soon enough - my DS is almost 5 months & he's now officially over his play mat lol!!
DS enjoyed being sung to especially songs with actions such as Incy Wincy Spider etc. He also loved me making funny sounds or funny faces, oh geez he'd laugh his little head off!!
Maybe also some books? Board books are a great idea, ones with just pictures on the pages. You can make up tons of stories & she'll just love listening to your voice. I usually lie us both on the floor & hold it up so we can both see it.
Those links are great too!! I didn't think DS would find them very interesting but bought some anyway. Well he just loved them - holds them, chews on them, throws them around.
Have fun!!