Summary - Northwest of Trout Lake, WA, Flattop Mountain is the site of a microwave tower and a couple of repeaters. While it is a driveup, the last section of road is best negotiated in a 4WD vehicle with good tires. There are no views and little open space on the tree covered summit. As with other hills in this area, ATT cellular and APRS coverage are both very good while making 2M contacts is hard. Flattop is not on my peaks-I-must-visit-againlist, though I would probably include it in a multiple day peak excursion.
TL;DR - I visited Flattop the same day I activated Peterson Ridge. The summit is broad, tree covered, and without views except along the power line that drops down the steep east side of the mountain. While it is not a summit for a leisurely activation, you can get up and down it in a couple of hours, making it a good candidate for a multi-peak day. There is a large microwave site, along with two small repeater buildings, on the summit but I noticed little RF interference. I didn't have any luck on 2M; HF contacts were easy and there are plenty of trees from which to hang a wire.
Google maps will suggest driving up the power line trail, which I don't think is possible, even on a motor bike. Instead take WA141 through Trout Lake to FS24 past the Peterson Campground (where it turns north) to FS8821 to FS091. If you want another hill to do, Peterson Ridge is nearby and an easy bushwhack. While 24 is wide and well maintained, the road gets progressively more treacherous. The road up to the summit, FS091 is single lane, without turnouts, and steep in places; it is not a road I would do with good tires and 4WD and I was glad I didn't encounter another vehicle while on it.