You might want to add a little info on where you are. Is it a Pacific NW zone 8 or a Louisiana 8?

I lived in Texas for a couple years and my lawn just kinda did this on it's own. Since the grass was dormant in the winter there were alot of winter 'weeds' that came and bloomed in the winter. Helps that I had a crappy lawn.

Spring beauty (claytonia) put on the best show, dead nettle seeded in, some wild allium, some wine cups (Callirhoe) and some bluebonnets were starting to establish. They didn't love the mowing, but as long as I let them seed and gave them a couple extra weeks to grow in the spring they did ok.

I would think bulbs such as scilla siberica, grape hyacinths, crocus, fritillaria meleagris, and ipheion would be worth a try.... there are probably some other more southern bulbs that I'm not familiar with.

Good luck. I'm trying to do something similar here with a part of my back lawn. Mostly crocus, cyclamen, fritillaria, smaller daffs, colchicum, winter aconite, and whatever other tiny bulbs make it. The grass is cut late July and then kept mown during the summer and fall. We'll see how it goes.

You say "I think crocus would be OK because they probably won't come up until after he's done with frequent mowings for the year"... But remember that the bulb foliage also needs to be able to die down naturally at the end of their blooming period. Would your Dad want to start mowing again before that happened? Especialy if you have a large variety of different bulbs there would be foliage going on for months. Personally I love this style of bulb planting but you have to be prepared to have patches of long grass at certain times of the year. It can look lovely with mown paths meandering through a meadow.

My parents are in the Dallas area, so a TX zone 8.

When do crocus plants send up leaves? I think maybe small clumps are the best solution because my dad will mow ruthlessly if things are too spread out.

I want to seed their front yard with bluebonnets.

I think my dad thinks the city is going to make him mow no matter the plant if they get too high. I think if I put in shorter growing plants, especially in the middle area, it would be fine. I could reserve larger plants for the outside edges.

I just don't want to waste my time.

Ah ha. I was about 45 minutes south of Dallas, near Corsicana.
If you stick to the earlier bloomers they will be done with flowers and growth before it gets hot enough for the st aug grass to wake up. If you go with the bluebonnets, might as well throw in a couple of the pink evening primrose. The primrose wouldn't even mind being mowed a couple times here and there when they get a little ragged.

If you can cruise the older neighborhoods or check out craigs list or freecycle I bet you could find someone willing to share some hardier amaryllis, criniums, red spider lilies, oxblood lilies..... not all of which would work out in the lawn, but all are cool bulbs that I miss seeing here up north.

Oh and sternbergia lutea (had to look that one up) looks like a yellow winter crocus that I remember seeing in older cemetaries etc. very nice and probably good for a lawn with winter leaves and late winter flowers.

Keep the edges mown along the street, neighbors and driveways and it looks like you are doing it on purpose and not just making a mess.

If you're interested, here is a blog about a texas yard's transformation from lawn to flowery prairie. Check out the archives and you can follow it from inspiration to completion. The whole thing might be way beyond what you have in mind, but he gives some tips on how to keep the place neat in regards to city ordinances.

Here is a link that might be useful: prairie in Plano