Hm...you can show her some flash cards using the computer. There are a number of places that you can download them from.
What about reading to her or singing nursery rhymes? For reading, you dont't have to buy special books...just read whatever that's in the house. Eva loves to stare at me when I'm reading so I'll read outloud to her and it entertains her for a while. Sometimes she'll laugh and start babbling as well. I know my daugther loves it when we do that (sing songs and rhymes) and dance with her as well. She also likes to stare at herself in the mirror.
And our cheapest "toy" to date? The great big outdoors! Whenever she's tired of being in the house, I put her in her stroller and take her around the neighbourhood.